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Full text of "Fluorocarbons & their Derivatives"

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Fluorocarbons and their Derivatives 

R.E.Banks 



University Chemistry Series 



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Macdonald 
Technical & Scientific 



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Fluorocarbons 

and their 

Derivatives 



By 

R. E. BAN K S, b. sc, ph. d., f. r. i. c. 

Senior Lecturer in Chemistry, The University of 
Manchester Institute of Science and Technology 




MACDONALD TECHNICAL & SCIENTIFIC 

London 



MACDONALD & CO. (PUBLISHERS) LTD 
St. Giles House, 49/50 Poland Street, London, W. 1 



© R.E.Banks 1970 



First edition 1964 
Second edition 1970 



SBN 356 02707 4 Cloth Bound Edition 
SBN 356 02798 8 Paper Bound Edition 



All rights reserved. No part of this publication 
may be reproduced, stored in a retrieval system, 
or transmitted, in any form or by any means, 
electronic, mechanical, photocopying, recording 
or otherwise,, without the prior permission of 
Macdonald and Co. (Publishers) Ltd. Printed in 
Germany by Lefpziger Druckhaus, Leipzig. 



PREFACE TO THE FIRST EDITION 

I hope that this book will prove both valuable and interesting to many 
chemists, including advanced undergraduate students, who want to learn 
about a branch of chemistry which attracts considerable academic and 
industrial attention. It should be particularly useful to those who intend to 
undertake research in fluorocarbon chemistry as a new venture. It will 
also serve as a reference work for those already engaged in such research 
or involved in industrial applications of fluorocarbon compounds and re- 
lated materials. Much factual material is presented, preparative methods 
are emphasized, possible reaction mechanisms are given where it is expe- 
dient to do so, and a large number of original publications are referred to. 

I want to thank Professor R. N. Haszeldine for his interest and encour- 
agement, Dr. Henry Heal for counsel, and the staff of Oldbourne Press for 
their friendly assistance. 

October, 1964 Ronald Ebio Banks 



PREFACE TO THE SECOND EDITION 

This new edition has been prepared with two main objectives in mind: 
one, to bring up to date the first edition, written during 1963; and two, 
to extend the coverage of organic aspects of the subject. Considerable 
headway has been made in fluorocarbon chemistry since 1963, making it 
possible now to introduce, for example, sections concerned with allenes, 
ketenes, diazirines, olefin oxides, and pyridines and to extend many of the 
original sections. The natural increase in the organic content thus achieved 
has been amplified by the inclusion of a section on perfluoroketone chem- 
istry. Finally, an appendix containing infrared and nuclear magnetic re- 
sonance data has been added, mainly for the benefit of practising fluoro- 
carbon chemists. 

I am deeply indebted to my friend and colleague Dr. M. G. Barlow for 
his valuable contribution to the appendix. 

R. Eeic Banks 



CONTENTS 

Chapter Page 

1 Introduction 1 

2 Aliphatic Fluorocarbons 7 

3 Derivatives of Perfluoroalkanes 70 

4 Perfluoroalkyl Derivatives of the Elements 102 

5 Perfluorinated Aromatic Compounds 203 

Appendix 

Spectroscopic Properties (i.r. and n.m.r.) of Fluorocarbon 

Compounds 230 

Index 239 



Chapter 1 
INTRODUCTION 

I. DEFINITIONS 

Fluorocarbons are compounds containing only carbon and fluorine ; they 
have structures analogous to the familiar hydrocarbons and are the parent 
compounds of a relatively new branch of organic chemistry called fluoro- 
carbon chemistry. Theoretically fluorocarbon derivatives arise through 
substitution of fluorine in fluorocarbons by functional groups that are 
mostly associated with organic, organometallic, and organometalloidal 
chemistry. No fluorocarbon compounds have been found in nature. 

II. HISTORY 

The only fluorocarbons definitely characterized and reported in the 
literature before 1937 were carbon tetrachloride, CF 4 , hexafluoroethane, 
C a F 6 , and tetrafluoroethylene, CF 2 :CF a . This situation stemmed mainly 
from the difficulties and dangers associated with the preparation and use 
of fluorine in the early years of fluorine chemistry. 

The first report of a fluorocarbon appeared in 1890 when Moissan, the 
French chemist who isolated fluorine in 1886, claimed that he had separated 
carbon tetrafluoride from products obtained by igniting finely-divided 
carbon in fluorine. 1 The boiling point of Moissan's material (— 15°) is so 
grossly incorrect, however, that credit for the isolation of carbon tetra- 
fluoride must go to his countrymen Lebeau and Damiens. Initially, in 
1926, they separated this fluorocarbon from fluorine liberated at a carbon 
anode during the electrolysis of fused beryllium fluoride and placed its 
boiling point near to — 150° ; later, in 1930, they described the isolation of 
a sample with b.p. —126° (—128° is the currently accepted value) from 
products obtained by the direct fluorination of carbon. 8 ' 3 Pure carbon 
tetrafluoride was also obtained in 1930 by the German chemists Ruff and 
Keira by careful distillation of products arising from the fluorination of 
wood charcoal. 4 

Products with boiling points higher than that of carbon tetrafluoride 
were obtained by Lebeau and Damiens and by Ruff and Keim from the 
fluorination of carbon. Both sets of workers believed that these contained 
higher homologues of carbon tetrafluoride, but were unable to prepare 



Introduction 

sufficient material for proper investigation because of the frequent and 
often violent explosions that occur when fluorine is brought into contact 
with carbon. Investigations by Ruff and Bretschneider threw some light 
on the cause of these explosions; 5 they discovered that Norite and graphite 
absorb fluorine under certain conditions to yield the so-called carbon mono- 
fluoride, (CF)„, a graphite intercalation compound that decomposes vio- 
lently when rapidly heated to give a cloud of soot, carbon tetrafluoride, 
and higher fluorocarbons. 

Pure hexafluoroethane, b.p. —78°, was first isolated in 1930 by the 
Belgian chemist S warts from the products of electrolysis of aqueous tri- 
fluoroacetic acid. 6 His identification of this fluorocarbon was confirmed 
in 1933 by Ruff and Bretschneider, who decomposed carbon tetrafluoride 
in an electric arc and obtained both hexafluoroethane and tetrafluoro- 
ethylene; 7 earlier claims that tetrafluoroethylene had been prepared by 
heating tetrachloroethylene with silver fluoride 8 and by direct fluorination 
of charcoal at low temperatures 9 can be ignored. 10 A second method of 
preparation of tetrafluoroethylene, namely dechlorination of 1,2-dichloro- 
tetrafluoroethane, CF g Cl.CF 2 Cl, with zinc, was disclosed in 1934. 11 The 
halogenoethane was prepared by heating hexachloroethane with the mixed 
antimony halide SbF 3 Cl 2 , a method based on work of Swarts on the pre- 
paration of organic fluoro-compounds by the technique of halogen exchange 
between antimony trifluoride and organic chlorides, bromides, and iodides. 12 
Swarts' studies, begun in 1891, led to the introduction of dichlorodifluoro- 
methane, CF 8 C1 4 , by Midgley and Henne in America during 1930 as an 
inert, non-toxic refrigerant, which had such outstanding advantages over 
other types of refrigerant that it rapidly became a major industrial chemi- 
cal. 13, 14 A number of simple chlorofluoro-methanes and -ethanes are now 
manufactured on a large scale in many parts of the world and, in addition 
to their original use as refrigerants, they find wide application as aerosol 
propellants for insecticides, paints, and a multitude of toilet preparations. 

In 1937 the American chemists Simons and Block discovered that 
mercury promotes smooth reaction between fluorine and carbon at tem- 
peratures just below dull red heat, and from the product they separated 
carbon tetrafluoride, hexafluoroethane, and the new fluorocarbons C 3 F g 
(b.p. —38°), C 4 F 10 (two isomers, b.p. —4-7° and 3°, respectively), cyclo- 
C 5 F 10 (b.p. 23 °), cyclo-C 6 F 12 (b.p. 51°), and cyclo-C 7 F 14 (b.p. 80°) ." These 
fluorocarbons were found to be chemically and thermally very stable, and 
their surprisingly low boiling points were attributed to weak intermolecular 
forces. The important fact that emerged from this work was that open and 
closed chains of CF 2 groups are stable, and this led to the realization that 
all structures associated with saturated hydrocarbons should be capable 
of duplication in terms of carbon and fluorine. At that time (1939), how- 
ever, it must have appeared unlikely that synthesis of a range of fluoro- 
carbon structures would be achieved in the near future ; although it should 



History 

be mentioned that Bigelow and his co-workers in America were steadily 
developing techniques for moderating the violence of reactions between 
fluorine and organic substrates and thus laying the foundations of a standard 
method of preparation of fluorocarbons. 16 

Events soon occurred that completely changed the picture and gave an 
impetus to fluorine chemistry that has remained to the present day. 

In 1940 materials were needed for use as buffer gases, coolants, lubricants, 
and sealants in chemical plant handling highly reactive uranium hexa- 
fluoride, the only volatile uranium compound available for use in a gaseous 
diffusion process for concentrating the ^U isotope required for the develop- 
ment of atomic bombs. The discovery that a fluorocarbon sample prepared 
by Simons was inert to uranium hexafluoride led to an intensive and suc- 
cessful search by American chemists and technologists for methods of pre- 
paration of saturated fluorocarbons that could be applied on an industrial 
scale ; commercial methods of preparation of fluorine and the highly inert 
polymers polytetrafluoroethylene and polychlorotrifluoroethylene were also 
developed. 17 - 18 Some of the fluorocar bonsprepared during the years of the 
Second World War are shown below: 



CF 3 .CF 2 .CF 2 .CF 2 .CF £ .CF 2 .CF 3 (CF 3 ) 3 C.CF 2 .CF(CF 3 ) 2 CF 2 :CF.CF:CF 2 
b.p. 82° b.p. 104° b.p. 6° 



CF 3 

j Fg Fg F 2 F 2 

F S C- CF 2 F 2 C F X!F 2 F c FC -CF CF 2 

F 2 C X /CF 2 F 2 (X V /CF 2 X C— (T X C— C x 

C C Fa F2 Fs Fa 

b.p. 22° b.p. 76° m.p. 75° 



F F F 

2 Jg i?2 *2 -^2 

FjC-^ ^C X X CF 2 FjCK X C / ^C^ X CF 2 

III I I I I 

X C / I ^*cr ^Cr I ^Cr I v cr 

FTP T2 "C* TO 

2 I J> 2 i>2 I r 2 I x 2 

F . F F 

b.p. 140° m.p. 76-81° 



When the details of these spectacular wartime developments were revealed, 
it was clear that a new branch of organic chemistry based on fluorocarbons 
as the parent compounds could be established provided methods of syn- 



Introduction 



thesis could be found for fluoroearbon derivatives Rj>.X, where Rj is a 
fluorocarbon group, e.g., 



CFg.LCF2J»*CF2~ 



CF a — CF— FaCT X CF— 

II. I I , 

C 



V: 



2 



and X is a functional group, e.g., — C0 2 H, — CHO, — OH, — SH, — S0 3 H, 
—NO, — -N0 2 , — NH 2 , — Mgl. It was also realized that substitution of 
hydrogen or chlorine for some of the fluorine atoms in the R F groups would 
undoubtedly give stable structures, so that the scope for investigation was 
unparalleled in chemistry. 

III. RECENT DEVELOPMENTS 

During the past twenty years much academic and commercial interest 
has centred around fluorocarbon chemistry. As a result, fluorocarbon 
analogues of members of nearly all the homologous series of organic chem- 
istry are known and fluorocarbon derivatives of many elements have been 
prepared. In addition, many fluorohydrocarbon derivatives have been de- 
scribed. Academic interest in fluorocarbon derivatives has been stimulated 
by the challenge of finding methods of synthesis and by the differences 
that often exist between these compounds and their hydrocarbon counter- 
parts. Commercial interest in fluorocarbon chemistry is still based mainly 
on the high chemical and thermal stability of fluorocarbon systems and 
does much to encourage research in this area of chemistry. 

This book is intended to provide a fairly up-to-date, but not exhaustive, 
discussion of fluorocarbon chemistry that should prove suitable for chemists 
at all levels from final-year undergraduates upwards. In the section on 
fluorocarbon derivatives emphasis has been placed on compounds con- 
taining the various elements, since it is in this area of the subject, which 
lies between organic and inorganic chemistry, where a great deal of novelty 
is found. 

IV. NOMENCLATURE" 

For purposes of nomenclature fluorocarbons and their derivatives are 
treated as hydrocarbon derivatives. The number of fluorine atoms may 
be indicated in the names of compounds, e.g., octafluoropropane (C 3 F g ), 
pentafluoropropionic acid (C 2 F s .C0 2 H), but in many cases these are cum- 
bersome and do not immediately reveal that the compounds described are 
fluorocarbon in nature. Thus use of the prefix 'perfluoro' in conjunction 
with basic hydrocarbon nomenclature is often preferred. The term 'per- 
fluoro' denotes substitution of all hydrogen atoms attached to carbon 



Nomenclature 

atoms except those whose substitution would affect the nature of the func- 
tional groups present; for example, perfluorobutyraldehyde is C 3 F 7 .CHO and 
not C 3 F 7 .COF, which clearly is an acyl fluoride, perfluorobutyryl fluoride. 
'Perfluoro' may refer to the whole word or to part of the word to which 
it is attached, but not to more than one word. Parentheses should be used 
where necessary to avoid ambiguity as to whether 'perfluoro' refers to 
only part of a name or to a whole name. Examples of the use of the 'per- 
fluoro' nomenclature are given below. Since, in the literature, the structures 
of fluorocarbon compounds are described by the use both of Greek or Latin 
numeral roots and of the prefix 'per' together with 'fluoro' and basic hydro- 
carbon nomenclature, no attempt has been made to standardize on one 
or the other type of nomenclature in this book. 



perfluoro -n -hexane 

perfluoropropene 

perfluoroacetaldoxime 

perfluoropiperidine 



Examples: 


CF 8 


.[CFJ..CF, 


CF 3 


.CF:CF 2 


CF a 


.CH:N.OH 




A^ 


Fi T 


iF 2 


fL 


I 




1 
H 




/K/* 


f 


\yv % 


F 2 k 


F 2 




CF:CF 2 



*t 


F 2 




F 4 C /Cx C xUx CF- 

1 1 1 


CF, 


1 ! 1 




P 2 


1 P 8 
F 





perfluoro(methyloyclohexane) 



(perfluorovinyl)benzene 



perfluoro(decahydro-2-methylnaplithalene) 



This use of the prefix 'perfluoro' excludes names in which 'perfluoro' is 
preceded by other prefixes. Thus the compound CF 3 .CFBr.CF 3 is called 
perfluoro-2-bromopropane(or2-bromoheptafluoropropane) and not 2-bromo- 
perfluoropropane. This limitation is imposed to avoid the implication that 



Introduction, 

some atom other than hydrogen has been substituted. For this reason also, 
the convenient term fluorocarbon hydride should strictly not be used, as 
in Chapter 3, to define compounds of the type R F H, where R F = perfluoro- 
alkyl (e.g., CF 3 ) or -cycloalkyl (e.g., cyclo-C 6 F u ). A convenient way to 
define the structure of a polyfluoro-compound containing hydrogen not 
associated with a functional group is to use a prefix consisting of the num- 
bers or symbols allocated to the carbon atoms bearing the hydrogen atoms, 
each followed by the letter H; according to this nomenclature the com- 
pounds CF 8 .CHF.CHF 2 and CHF a .CF a .CF a .CF 2 .CO a H, for example, are 
called li7,217-hexanuoropropane and &H-octafluorovaleric acid (or 5#-octa- 
fluoropentanoic acid), respectively. Compounds named according to this 
method must comply with the rule that ordinarily the number of hydrogen 
atoms not associated with the functional group must not exceed four and 
the ratio of such hydrogen atoms to fluorine atoms likewise not part of a 
functional group (e.g., — COF, — S0 2 F) must be not greater than 1:3. 

BEFEBENCES 

1. Moissan, Compt. rend., 1890, 110, 951. 

2. Lebeau and Damiens, Compt. rend., 1926, 182, 1340. 

3. Lebeatt and Damiens, Compt. rend., 1930, 191, 939. 

4. Buff and Keim, Z. anorg. Chem., 1930, 192, 249. 

5. Buff, Bretschsteider, and Ebbbt, Z. anorg. Chem., 1934, 217, 1; Croft, 
Quart. Rev., 1960, 14, 1. 

6. Swarts, Bull. sci. acad. roy. Belg., 1931, 17, 27. 

7. Btjff and Bretschneider, Z. anorg. Chem., 1933, 210, 173. 

8. Chabbie, Compt. rend., 1890, 110, 281. 

9. Humiston, J. Phys. Chem., 1919, 28, 572. 

10. Booth, Btjrchfield, Bixby and McKei/vey, J. Amer. Chem. Soc, 1933, 65, 2231. 

11. Locke, Brode, and Henne, J. Amer. Chem. Soc., 1934, 56, 1726. 

12. Kauffman, J. Chem. Educ, 1955, 32, 301. 

13. Hamilton, in Advances in Fluorine Chemistry, ed. Stacey, Tatlow, and Sharpe, 
Butterworths, London, 1963, Vol. 3, p. 117. 

14. Garrett, J. Chem. Educ., 1962, 89, 361. 

15. Simons and Block, J. Amer. Chem. Soc, 1937, 59, 1407; 1939, 61, 2962. 

16. Bigelow, Chem. Rev., 1947, 40, 51. 

17. Various authors in Ind. Eng. Chem., 1947, 39, 236-433. 

18. Various authors in Preparation, Properties and Technology of Fluorine and 
Organic Fluoro Compounds, ed. Slesser and Sohram, McGraw-Hill, New York, 
1951. 

19. Handbook for Chemical Society Authors, Special Publication No. 14, 1960, p. 191. 



Chapter 2 
ALIPHATIC FLUOROCARBONS 

I. PERFLTJORO-ALKANES AND -CYCLOALKAN E S 

A. Preparation 

Three methods are available for converting hydrocarbons or their deriv- 
atives into saturated fluorocarbons : direct vapour-phase fluorination, in- 
direct fluorination with certain high- valency metal fluorides, and electrolysis 
in anhydrous hydrogen fluoride. 

1. Direct Vapour-phase Fluorination of Hydrocarbons. 1 Eeaction of a 
hydrocarbon with fluorine proceeds via a free-radical chain mechanism of 
the type associated with photochlorination; progressive replacement of 
hydrogen and saturation of any multiple bonds or aromatic systems by 
fluorine occur, and ultimately a perfluoro-alkane or -cycloalkane is formed, 
e.g., 

CH 4 »• HF+CHj- '-*■ CHjF+F- ► etc. > CF 4 

These reactions are highly exothermic, since heats of formation of G— F 
and H— F bonds are high (ca. 105 and 135 kcal/mole, respectively) and 
the bond dissociation energy of fluorine is only 37 kcal/mole [cf. D(C1— G) 
= 57 kcal/mole] : 

>CH— +F 2 — ♦• >CF— + HF; Jtfss - 104 kcal (cf. JHfor chlorination » - 24kcal) 
>C:C<+F 2 — *■ >CF.CF<; zlH« - 1 10 kcal {cf.AH for chlorination *- 33 kcal) 

and unless the heat liberated is rapidly dispersed, combustion and extensive 
fragmentation of the carbon skeleton occur. At elevated temperatures, 
fluorine atoms necessary for initiation of chain reactions can obviously arise 
by thermal dissociation of molecular fluorine : 



However, fluorine attacks hydrocarbons even at low temperatures, in the 
dark, and in the presence of inert diluents; and it has been suggested that 
under these circumstances fluorine atoms are formed, together with alkyl 



Aliphatic Fluorocarbons 

radicals, when an energetic fluorine molecule collides with a hydrogen atom 
of a hydrocarbon chain: 2 

-H+F, 



/ 



-* -^C-+HF+F- 



Such a reaction is favoured thermodynamically by the low bond dissociation 
energy of fluorine and the very high bond dissociation energy of hydrogen 
fluoride. 

In general, the direct fluorination of a hydrocarbon is best effected by 
mixing the reactants, both heavily diluted with nitrogen, in a heated steel 
tube packed with a 'catalyst' of gold- or silver-plated copper turnings 
(see Fig. 2.1); the main function of the packing is to disperse the heat of 




<N t 



HHf| 



Fig. 2.1. Direct fluorination apparatus.* {By courtesy of The Chemical Society.) 



F. Silica tube furnace for heating the hydro- 
carbon reservoir. 

A and B. Traps for the condensation of polymeric 
material. 

L. Copper tube dipping into 'Cerechlor' or a 
fluorinated oil to act as a safety valve. 

K. Potassium fluoride scrubber (removed during 
preparation of high-boiling fluorooarbons). 

P. Two copper XT-tubes joined in serieB and coo- 
led by a mixture of solid carbon dioxide and 
alcohol. 

W. Liquid-air trap to condense any volatile de- 
composition products. 



ft and It. Pyrex flow-meters. 

M. Mixing chamber for fluorine and nitrogen. 

S. Copper spiral for preheating the fluorine - 

nitrogen mixture. 
R. Steel reaction vessel ( 34 in. x 3 in.) filled with 

gold-plated copper turnings. 
Ti. Thermometer pocket. 
B. Nickel baffle' plate, extending »/» length of R; 

lower half drilled with Va2 in. holes V 2 in. 

apart. 
H. Furnace for preheating hydrocarbon-nitrogen 

mixture. 
V. Copper-glass ground Joint. 
O. Graduated 60-c.c. Pyrex vessel carrying a 

thermometer, T 2 . 

reaction, although some fluorination doubtless occurs via intermediate 
formation of surface films of gold or silver fluorides. This method, called 
the 'catalytic' method.'was studied extensively during the early 1940's by 
Cady and Grosse at Columbia University and by Smith, Musgrave, and 
Haszeldine at Birmingham University; details of some of their experiments 
are given in Table 2.1. The yield of required fluorocarbon decreases as the 
molecular complexity of its hydrocarbon precursor increases, and it is diffi- 
cult to fluorinate hydrocarbons above C 10 without extensive decomposition 
occurring. Alicyclic compounds appear to give higher yields of perfluoro- 
cycloalkanes than aromatic compounds; and partially-fluorinated com- 



m-,* 



(a) Electrochemical fluorination apparatus in the 
Chemistry Department of the University of Manchester 
Institute of Science and Technology. 

1. Nickel electrochemical fluorination cell (see Plate lb); 

2. nickel condenser (coated with ice) cooled to ca. — 20° with 
a refrigerated glycol— water mixture; 3. heated (100°) mild 
steel tubes containing sodium fluoride and rubber chips to 
remove any hydrogen fluoride and fluorine oxides, respec- 
tively, from the hydrogen stream carrying the volatile fluori- 
nation products., to the Pyrex traps 4. and 5. [the steel- 
encased Dewar vessels containing alcohol— solid CO % ( — 72°) 
and liquid oxygen (— 183°) respectively have been lowered 
for the purpose of this photograph]; 6. electrical controls; 
7. pump for circulating the glycol— water mixture, which is 
cooled to ca. — 20° in a refrigerated tank 8. 



(b) Laboratory electrochemical fluorination cell. 

1. 1 l.-Capacily nickel cylinder; 2. puck of alternate nickel 
anodes and cathodes insulated from each other by polytetra- 
fluoroethylene washers; 3. condenser; 4. and 5. inlet and 
outlet, respectively, for refrigerated glycol-water mixture; 

6. inlet for anhydrous hydrogen fluoride and organic solute; 

7. exit for hydrogen and volatile fluorocarbon products; 

8. nickel valve; 9. polytetrafluoroethylene gasket. 



PLATE 1 




(a) Model showing the helical configuration adopted by a polytetra ftuoroethylene chain to relieve 
over -crowding of fluorine atoms. The repeat distance of 16-8 A for 13 CF 2 groups is indicated by 
the Angstrom callipers. 




(6) Model of a segment of a polyethylene chain with a fully staggered conformation. 



PLATE 2 Courtauld molecular models of polytetrafluoroethylene and polyethylene. 



{Photos, by courtesy of Griffin i: Oeorgt Ltd., Wembley, Middlesex.) 



Perfluoro-Alkanes and -Cycloalkanes 
Table 2.1. Examples of the 'Catalytic' Fluorination ot Hydrocarbons 5 - 6 



Be action 
temp. 
Hydrocarbon 'Catalyst' (°C) 



Yield 
Fluorocarbon product (%) 



n-C 7 H 16 



^X 



Ag— Cu 



Ag— Cu 



CF 3 



Ag— Cu 



CH 3 

I 



H a C 



Au— Cu 



S CH 3 




Au— Cu 




Ag— Cu 



135 



265 



200 



200 



370 



300 



n-C,F 16 



F 2 

FjC-^ X CF 2 


1 1 
F 2 C\ _^CF 2 


F. 


CF 3 

| 


/-C v 

FjC-^p -CF 2 

II 


1 1 
FjC\ _/CF 2 


P 2 


CF 3 


/Ox 

FiC^p V CF 2 

1 1 


1 1 

F 3 C / F "^C / F ^CF 3 
P 2 


F 2 P F 2 
F 2 C / ' ^C' X CF 2 


F2C -c/?^c^ CF2 



F 2 C 



F 2 F F 2 

F 2 F F 2 F F 2 



F 2 C 



CF 2 



CF 2 



\ /Lv\ /V/\ /ti! 2 

^C^ | -c' i ^c-^ 

F 2 j F 2 p F 2 



62 



58 



85 



11 



19 



43 



pounds, obtained by simple halogen-exchange methods not involving 

fluorine, e.g., 

CC1 3 CF 3 




I 



+ 3HF 



+ 3HC1 



give much better yields than the parent hydrocarbons. 

2 



Aliphatic Fluorocarbons 

The cobalt fluoride Method described below is preferred to the 'catalytic' 
method for preparation of saturated fluorocarbons, and the latter is now 
used only in academic studies on the fluofination of hydrocarbon deriva- 
tives. Bigelow and his co-workers have recently developed unpacked 
fluorinators with special inlet jets for reactants, which promote mild 
fluorination, and with these they have prepared the fluorocarbons C 2 F 6 , 
C 3 F 8 , n-C 4 F 10 , and (CF 3 ) 3 CF in high yield from the corresponding gaseous 
hydrocarbons. 3 News of the application of this technique to high-boiling 
hydrocarbons is awaited with interest. 

2. The Cobalt Fluoride Method of Fluorination. This method was developed 
by Fowler and his collaborators at Johns Hopkins University, Baltimore, 
in 1941 ; it is much more convenient than the 'catalytic' method and, in 
general, gives better and more reproducible yields of saturated fluoro- 
carbons. 4 

In the cobalt fluoride method, saturated fluorocarbons are prepared by 
passage of the vapour of an aliphatic or aromatic hydrocarbon over a heated 
bed of cobalt trifluoride, e.g., 

C0F3, 300° 
n-C 6 H 12 > n-CtFutiSX) 

CoF 3 , 225-350° 
n-C 7 Hi 6 > n-C,F 16 (80%) 

CoF a , 300-400° , „, , 

n-C M Ha4 *■ n-C 16 F 84 (29%) 

C*H 5 f aFfi 

^N CoF„300° *f/^CF 2 

► I I (59%) 

FjO CF 2 



CH a 



CoF, , 300° 



CF» 



FjCKj-^CFa 



> I I (50%) 

NjH. F2<K C / f X CF 3 



F 



2 



F 2 F 2 F 2 F 2 

/ == \ r„ TTl / == \ CoF 3 ,340° / C "~°\ r , /' 

I )— [CH 2 ] 2 — i ) >- F 2 C FC— [CFja— CF 



C— C^ 



CF 2 (66%) 



F 2 F 2 F 2 F 2 

During fluorination the cobalt trifluoride is reduced to cobalt difluoride; 
further reaction of the difluoride with- fluorine in the same apparatus 
regenerates cobalt trifluoride, so that passage of hydrocarbon vapour and 

10 



Perfluoro-Alhanes and -Oycloalkanes 

of fluorine alternately over a heated bed of powdered cobalt trifluoride 
enables the process to be essentially continuous : 



>CH-+2CoF. 



-> >CF— +HF+2CoF 2 : AHw — 58 kcal (cf. AH for direct 

fluorination. s» — 104 kcal) 



<.„ r. ^ 200-800° 
2CoF a +F 4 ► 



2CoF, 



Considerably less heat is liberated during the fluorination of a hydrocarbon 
with cobalt trifluoride than with elemental fluorine, so provided the con- 
ditions are carefully controlled, fluorination can be effected with this 
reagent without extensive C — C bond fission. 

A simple laboratory cobalt fluoride reactor consists essentially of an 
electrically-heated copper or mild steel tube containing layers of cobalt 
trifluoride spread on trays ; however, the efficiency of the method can be 
improved by use of a reactor fitted with a coaxial stirrer to agitate the 
fluorinating agent (see Pig. 2.2): 7 Horizontal gas-heated reactors containing 




JQ. 



A J 



3^ESS« 



D A J B 

Fig. 2.2. Laboratory cobalt fluoride reactor. 



Tl 



A. Electrically-heated copper, mild steel, or 

nickel tube (4 ft. long x 4 in. i.d.). 

B. Nickel paddles. 

C. Hollow stirrer shaft housing four thermo- 

couples to measure the reactor temperature 
at intervals along its length. 

D. Special nitrogen-swept bearing. 

E. Electric motor to rotate the stirrer at 5 rev/ 

min. 



F . Electrically-heated inlet tube into which li- 
quid hydrocarbon is metered so that it 
vaporizes before reaching the cobalt tri- 
fluoride J (ca. 6 kg). 

O. Exit tower containing baffle plates to pre- 
cipitate any cobalt di- or tri-fluoride ent- 
rained in the gaseous products. 

H. Exit line connected to cooled copper or mild 
steel traps. 



ca. 65 kg of stirred cobalt trifluoride were used in America during the 
Second World War to produce fluorocarbons for the Atomic Bomb Pro- 
ject. 8 

Silver difluoride, manganese trifluoride, cerium tetrafluoride, and lead 
tetrafluoride will also convert hydrocarbons into fluorocarbons at elevated 
temperatures, but they have not been so widely used as cobalt trifluoride. 4 

3. Electrochemical Fluorination. 9 ' 10 Many organic compounds, partic- 
ularly those containing polar groups, dissolve in anhydrous hydrogen 
fluoride to give conducting solutions. When such a solution is electrolysed 
at a low voltage (usually 5-6 V) so that free fluorine is not liberated, hy- 
drogen is evolved at the cathode and the organic solute is fluorinated at 



11 



Aliphatic Fiuorocarbons 

the anode, which must be made of nickel. This method of fluorination was 
discovered by Simons in 1941 ; it resembles fluorination with elemental 
fluorine or cobalt trifluoride since all hydrogen in organic compounds is 
replaced by fluorine, any multiple bonds or aromatic systems are saturated 
with fluorine, and fragmentation of carbon skeletons occurs, but, in general, 
functional groups (e.g., — COF, — S0 2 F) are retained to a much greater 
extent. Thus electrochemical fluorination is an extremely useful method 
of preparation of certain fluorocarbon derivatives from hydrocarbon-type 
compounds, as will be described later. The mechanism of electrochemical 
fluorination has not yet been established but may involve the formation 
of complex high-valent nickel fluorides. 10,11 

Hydrocarbons themselves are difficult to fluorinate electrochemically 
since they are only sparingly soluble in anhydrous hydrogen fluoride; the 
best result quoted in the literature is for the conversion of n- octane into 
perfluoro-n-octane : 

flIpcftroc2ifim ii^aI 

n-C s H.„ > n-CJ? 19 (ll%) plus small amounts of C, — C, fiuorocarbons. 

8 18 fluorination ! w /r * ' 

However, the principal products from electrochemical fluorination of many 
hydrocarbon derivatives, which are generally quite soluble in anhydrous 
hydrogen fluoride, are fiuorocarbons containing the same carbon skeletons 
as the organic groups in these precursors; thus perfluoro-n-octane is the 
major product when solutions of the compounds n-C 8 H 17 .OH, n-C 8 H 17 .NH 2 , 
n-C g H 17 .SH, and n-C 8 H I7 .C0 2 H in anhydrous hydrogen fluoride are electro- 
lysed (the last two compounds also yield small amounts of the fluorocarbon 
derivatives n-C 8 F 17 .SF 5 and n-C 8 F 17 .COF, respectively), and perfluoro- 
n-pentane is formed in at least 40 % yield during electrochemical fluorination 
of pyridine: 



t electrochemical 2 
fluorination 



1*2 F2 



(40%), (7%), Ci— C 4 fiuorocarbons, NF 3 



^ F 3 C CP 3 F 2 0\ /OF 2 

T 

F 
(I) 

The last reaction is the standard method of preparation of undecafluoro- 
piperidine (I). 

The apparatus required for electrochemical fluorination is relatively 
simple : it consists basically of an iron or, preferably, nickel cell containing 
a pack of alternate nickel anodes and cathodes and surmounted by a reflux 
condenser cooled to about — 20° to prevent serious loss of hydrogen fluoride 
(b.p. 19-5°). The perfluorinated products are either swept out of the cell 
by the cathodic hydrogen or, being dense and insoluble in hydrogen fluoride, 

12 



Perflvaro-Alkanes and -Oyeldalhanes 

accumulate at the bottom of the cell from whence they can be drained off. 
Laboratory cells [see Plates 1 (a) and 1 (6)] have capacities of 0-5-10 1 
and pass currents of 6-60 A at 5 V ; 10,000- A cells, 6 ft high x 4 ft diameter, 
are used commercially in America. 

B. Molecular Geometry of Perfluoroalkanes 

The existence of stable chains and rings composed of CF 2 units is made 
possible by the relatively small size of the fluorine atom (van der Waals 
radius 1-35 A; cf. hydrogen 1-2 A, chlorine 1-8 A) and the high strength 
of the C — F bond (see Table 2.2). However, the size of the fluorine atom is 



Table 2.2. Some Estimated Bond Dissociation Energies 13 



Bond broken 


D(R- 


-X) 


Bond broken 


D(R— X) 


R— X 




kcal/mole 


R— X 


kcal/mole 


CF 3 — F 




124 




CC1 3 — CI 


75 


C 2 F B — F 




123 




CH 3 — H 


101 


(CF S ) 2 CF- 


— F 


110 




CH 3 -F 


108* 


(CF 3 ) 3 C- 


F 


99 




CH 2 F— F 
CHF 2 — F 


115* 
120* 



* These D(C — F) values for fluoromethanes, which should be compared with 
D(CF 3 — F), are included to illustrate the strengthening of the C — F bond with in- 
creasing fluorine content. This effect and the accompanying marked contraction 
in C — F bond length can be discussed 18 * in terms of double bond-no bond resonance, 

e -g-> _ 

F F F 

I _l + 

F— C — F «->• FC=F •*-*■ F — C — F «-»• etc. 



F 



F 



F + 



This type of resonance stabilization is much more important in polyfluoromethanes 
than in other polyhalogenomethanes because structures involving halogen no-bonded 
as an anion contribute most when fluorine, the most electronegative halogen, is 
involved and halogen-carbon dative pa-Pn bonding decreases in the order F > 01 
>Br>I. 

such that a potential energy barrier of 4-35 kcal/mole hinders mutual 
rotation of the CF 3 groups in hexafluoroethane compared with a barrier 
of 3-0 kcal/mole to free rotation around the C — C bond in ethane ; 12 further- 
more, repulsive forces exist between fluorine atoms on alternate carbon 
atoms (1,3 F — F interactions) in perfluoro-n-alkanes which lessen the 
stability of these compounds and facilitate conformational isomerism. 

The operation of 1,3 F — F interactions in perfluorocetane, n-C w F 34 , and 
in polytetrafluoroethylene, [— CF 2 .CF 2 — ]„, is revealed by the results of 
X-ray diffraction measurements, 14 which show that the zigzag carbon 
chains in these compounds are not planar as in normal paraffins but are 

13 



Aliphatic Fluorocarbons 

twisted to relieve repulsions between fluorine atoms on alternate carbon 
atoms. Thus polytetrafluoroethylene has a helical carbon chain that under- 
goes a full 360° twist in 33-6 A, 13 zigzags, or 26 chain atoms [see Fig. 2.3 
and Plate 2 (a)]. 




Fig. 2.3. 14 Left: twisted zigzag chain found in perfluoroalkanes. 
Centre: perfltioroalkane molecule (side and end views). 
Bight: alkane molecule. (By courtesy of Nature.) 

Molecular models [see Fig. 2.3 and Plates 2 (a) and 2 (&)] demonstrate 
that a saturated fluorocarbon molecule is more streamlined than its hydro- 
carbon counterpart in the sense that the general shape is more nearly 
cylindrical and the projections and depressions in the profile are smaller 
in relation to the cross-sectional area. This close approach to cylindrical 
shape may be the simple explanation of the existence of disordered crystals 
in perfluorocetane and polytetrafluoroethylene (order-disorder transition 
points — 170° and 20°, respectively) at temperatures far below their melting 
points (125° and 327°, respectively), since rotations around chain axes 
and longitudinal translations would be facilitated by streamlined molecular 
shape. More important still, models reveal that the fluorine atoms in a 
saturated fluorocarbon form an almost impenetrable sheath which must 
give excellent protection from chemical attack to the carbon backbone. 

14 



Perflttoro-Alkanes and -Cyclpalkanes 

C. Physical Properties 

Saturated fluorocarbons posses a somewhat unusual ^set of physical 
properties. 16-1 ' They are colourless, dense, apparently non-toxic substances 
that solidify either as glasses or as soft waxy crystals; their melting points 
are usually higher than those of their hydrocarbon analogues (see Table 2.3). 
Due to low intermolecular forces the boiling points of saturated fluoro- 
carbons are much lower than would be expected from molecular weight 
values: perfluoro-n-pentane (M, 288; dj° 1-620 g/ml), for example, boils 
at 29-3°, whereas n-pentane (M, 72; df° 0-626 g/ml) boils at 36-2°. Inter- 
estingly, a close correspondence exists between the boiling points of satu- 
rated fluorocarbons and their hydrocarbon counterparts (see Table 2.3). 

Table 2.3. Melting Points and Boiling Points of Saturated Fluorocarbons and of their 

Hydrocarbon Analogues 



No. of carbon 
atoms 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


n -C„F 2 „ +s 
m.p. <°C) 


-184 


-106 


-183 


-85 


-125 


-86 


-80 




-16 


36 


b.p. (°C) 


-128 


-78 


-38 


-1 


29 


57 


82 


104 


125 


144 


m.p. <°C) 


-184 


-172 


-190 


-135 


-131 


-94 


-91 


-57 


-53 


-31 


b.p. (°C) 


-161 


-88 


-42 


-0-5 


36 


69 


98 


126 


151 


174 



CF, C 2 F 5 n-CgF, n-C 4 F, 

*"• I I ! I 

FjC^ X CF 2 F-jCK^F^-CFij FjC-^f^CFj F 2 C' f x CF 2 FjC^p^OFs 

F 2 0\ /CF 2 F 2 Cx /CF 2 F 2 C\ ,^CF 2 FjC x /CFj F S C\ ASF 2 



F* 


F 2 


F 2 


F 2 


F 2 


m.p. 58° 
b.p. 53° 


m.p. -38° 
b.p. 76° 


m.p. -60° 
b.p. 101-5° 


m.p. — 
b.p. 124° 


m.p. — 
b.p. 145 



CH, 





C 2 Hs 



n-C,H 7 



n-C«H 



m.p. 6-5 


m.p. -126° 


m.p. -129° 


m.p. —94-5° 


m.p. -79 


b.p. 80° 


b.p. 100° 


b.p. 129-5° 


b.p. 155° 


b.p. 177° 



The surface tensions and refractive indices of saturated fluorocarbons are 
extremely low, and the latter often lie below 1-3 (e.g., n-C 5 F 12 , Jijf 1-241; 
n-C g F M , wf> 1-2515), i.e. they are lower than those of any other type of 
organic compound. The absolute viscosities of saturated fluorocarbons and 
their temperature coefficients of viscosity are high in comparison with 



15 



Aliphatic Fluorocarbons 

those of saturated hydrocarbons; it has been suggested that this difference 
may be due to the relative stiffness of a fluorocarbon chain. 18 Saturated 
fluorocarbons have low dielectric constants, and the lowest ultrasonic 
velocity yet measured in a liquid at normal temperatures appears to have 
been recorded in perfluoro-n-heptane (444-0 m/sec at 60°). 

Among the most striking of the physical properties of saturated fluoro- 
carbons are their unusual solubility characteristics, which are of great 
value to physical chemists working on theories of solution. 19 Qualitatively, 
a fluorocarbon is generally a poor solvent, except for other substances 
with low internal pressures, e.g., other fluorocarbons, fluorocarbon deriva- 
tives, and some volatile inorganic fluorides (WF„, for example). Saturated 
fluorocarbons are practically insoluble in water, hydrogen fluoride, and 
alcohols, slightly soluble in hydrocarbons, more soluble in ether and chloro- 
carbons, and miscible with some partially -fluorinated hydrocarbons, e.g., 
benzotrifluoride, and chlorofluorocarbons, e.g., CF 2 C1.CC1 S . 

So far, investigation of the physical properties of saturated fluorocarbons 
has been hampered by the lack of pure compounds covering a range of 
structures. This is due partly to difficulties in synthesis and partly to 
difficulties encountered in the purification of saturated fluorocarbons. The 
methods of preparation discussed earlier yield products contaminated by 
lower homologues, isomers, and hydrofluorocarbons that are troublesome 
and often impossible to remove by precise distillation or fractional crystal- 
lization. A saturated fluorocarbon can be substantially freed from hydro- 
fluorocarbon impurities by ethanol extraction followed by treatment with 
silica gel, but it often tends to inter-distil with its homologues, and has a 
volatility almost identical with that of any isomer present (e.g., n-C 5 F 12 , 
b.p. 29-3°; i-C 5 F 12 , b.p. 30-1°). Isolation and purification difficulties' are 
not restricted to saturated fluorocarbons but are frequently encountered 
throughout fluorocarbon chemistry, and ihej are made more acute by the 
relatively small scale on which many experiments have to be carried out 
because of the high cost and limited availability of many starting materials 
and intermediates. Happily, most of these difficulties, and also analytical 
problems, can be overcome by gas-liquid chromatography; this technique 
was introduced as an analytical tool in 1952 by James and Martin and was 
adapted for preparative work by organic fluorine chemists in the mid- 
1950's. 20 A recent example of the use of preparative-scale gas-liquid chro- 
matography in fluorocarbon chemistry is the separation of perfluorodecalin 
into almost pure cis- and trans-forms, which boil only ca. 1° apart 21 (cis- 
and trans-decaiin boil 9° apart). 

D. Chemical Properties 

The outstanding feature of the chemistry of perfluoro-alkanes and -cyclo- 
alkanes is their great chemical and thermal stability. The factors responsible 
for this are the great strength of the C— F bond [e. g., a C— F bond in carbon 

16 



Perfluoro-Alhanes and -Oydoalkanes 

tetrafluoride has a bond dissociation energy, D(CF 3 — F), of 124 kcal/mole] 
and the shielding effect and high electronegativity of the fluorine atoms. 

Saturated fluorocarbons are unaffected by boiling concentrated acids and 
alkalis, and by oxidizing and reducing agents under normal conditions. 
They react with glass and silica at temperatures above 500° to yield carbon 
dioxide and silicon tetrafluoride, and with molten alkali metals to give 
carbon and alkali-metal fluoride; these reactions have been adapted for 
the elemental analysis of fluorocarbons : 2a 

°" 1000 ° >. ^±1 siF 4 (removed by NaF at 270°) + »C0 2 (weighed 

silica combustion 2 ; n 80 da-asbestos absorption tube) 
tube containing c ' 



C„F. 



n-Egn+3 — 



silica chips 
K, 650° 



> nC + 2(n + l)KF (dissolved in water and F~ estimated) 



in sealed nickel bomb 

Other active metals will also strip fluorine from fluorocarbons at elevated 
temperatures, and controlled defluorination of appropriate perfluoroalicyclic 
compounds with hot finely-divided nickel or, preferably, iron yields aromatic 
fluorocarbons, e.g., 23 

CF3 CF3 

I I 

F.C^CT, FC^CF 

I I T^iT I I (25%) 

F 2 C\ /CF 2 FC\ -#-CF 

F 2 F 

F 2 F F 2 F F 

FsO^^C^ ^CF 2 .. no FC-^ \C /C ^CF 

I I I £3 I II I <»*> 

, F 2 j 1 F 2 F F 

Saturated fluorocarbons are far more thermally stable than their hydro- 
carbon or chlorocarbon analogues (the latter are small in number), and 
their stability decreases with increasing chain length or chain branching 
(see Fig. 2.4). Carbon tetrafluoride is the most stable fluorocarbon: it 
survives temperatures of 1400-1500° during its preparation in almost 
quantitative yield from carbon, chlorine, and molten sodium fluoride, 24 
but it decomposes in an electric arc (temperature >2000°) to yield hexa- 
fluoroethane and tetrafluoroethylene. The thermal stabilities of all other 
saturated fluorocarbons are limited by the strengths of the C — C bonds 
present, which are much weaker than the C — F bonds [D(R r .CF 2 — CF 8 .Rj.) 
s» 86 kcal/mole; R F = perfluoroalkyl], and cleave homolytically to yield 

17 



Aliphatic FliAorocarbons 

initially fluorocarbon free radicals. The fate of the two free radicals formed 
by fission of the weakest C— C bond in a saturated fluorocarbon depends 
upon the circumstances. Co-pyrolysis of perfluorobicyclohexyl with toluene 



§ 2.000 



o 

8 '.soo 

3 1 1.000 

a • soo 



I 

^ 200 400 600 800 1000 

Heat of formation from atoms 
(Mcaiymole) 

Fig. 2.4. m Comparison of thermal stabilities. 
(By courtesy of the American Chemical Society.) 




4CI 4 C » Br « 



and with chlorine or bromine at 600-650° in a Pyrex tube yields undeca- 
fluorocyclohexane and the corresponding halogeno*-undecafluorocyclo- 
hexanes respectively : M . 



F 2 F 2 

y°- c \ 

F 2 C FC— — FC 



F 2 F 2 



F 2 F 2 
/C -C X/ F 



F 2 F 2 



600-650° 
CF 2 > 2F 2 C C 

F 2 F 2 F 2 F 2 



C4S5.CM3 



F 2 Fa 

/ C A/ F 



F 8 C 



c— c x 

F 2 F 2 



>F 2 C 



F 2 F 2 

/ \/ C,H|.CHV , 

C + C 6 H 5 .CH 2 ► (C 6 H 6 .OH 2 .) 2 



X3— C-^^H 
F 2 F 2 

F 2 F 2 

/°-V F 

(X-Clor >Fa ^ £ 

F 2 F 2 



X, 



F 2 F 2 
/O— C x /F 

+ X- »• F 2 C C 

F 2 F 2 



In the absence of a trapping agent, perfluorocyclohexyl radicals generated 
thermally become transformed into mainly perfluoro-(l,l-dimethylcyclo- 

* As used in this book, the term halogen does not include fluorine unless stated 
otherwise. 



18 



Perfluoro-Alkanes and -Cycloalkanes 

pentane) and perfluorocyclopentene, 2 '* ,1> possibly via initial isomerisation 
to perfluoro-(l-methylcyclopentyl) radicals and subsequent disproportiona- 
tion involving transfer of a CF 3 group. 27 b Similar reactions involving per- 
fluoro-n-alkanes require temperatures of 800-950°, e.g.,* 8 ' 89 

X,<X- CI or Br), 900-950° 
4 * silica tube 3 

Hat ca* 800° 

C 3 F e — > CH 2 F-, CHF 3 , C 2 F 6 H, C 

" carbon tube * 

Cla 800—900° 

n-C 5 F 12 " > CF.CL,, CF.C1, C,F 5 C1, n-C.F 7 Cl, n-C 4 F,Cl 

8 la silica tube a " s > a s > s 7 > 4 » 

The formation of methylene fluoride and dichlorodifluoromethane, re- 
spectively, in the last two reactions has been cited as evidence in support 
of the theory that a linear fluorocarbon free radical can decompose by 
elision of a CF 2 unit, 30 e.g., 

Rp.CFg.Rj' >■ Rf.CF 2 " -{-R^* ^ Rj? . CF 2 ^ ~H **f*-^ 



R r +CF 2 : C1 * > R F C1+CF 2 C1 2 

In the absence of a radical trap, C 3 — C 6 perfluoro-n-alkanes decompose in 
the region of 1000° to yield a mixture of saturated fluorocarbon(s), per- 
fluoropropene, perfluoroisobutene, a polymer resembling polytetrafluoro- 
ethylene, and trace of carbon, 85 - sl e.g., 

C,F 8 ftmamentatil050 '' > , C 2 F„(102), CF 3 .CF:CF 2 (008), (CF 3 ) 2 C:CF 2 (011), 
* 8 in Au-plated Cu vessel 2 * y "3 a\ i> an a\ 

[— CF 8 — ]»(0-31), C(trace)* 

A possible reaction path for this particular decomposition is shown below : 

1050° 
CF 3 .CF 2 .CF S * CF a .CF 2 .+CF 3 . 

CF 3 .CF 2 . > CF S .+ CF 2 : 

2CF 3 * C 2 F S 



CF 2 :CF 2 +CF 2 : - 
CF 3 .CF:CF 2 +CF 2 : >• (CF 3 ) 2 C:CF 2 



- [— CF 2 — ], 



Difluorocarbene is definitely liberated when the strained fluorocarbon 
perfluorocyclopropane is heated to about 170° and can be trapped with a 

* Figures in parentheses are moles of product per mole of C 3 F 8 decomposed. 

19 



AUphatic Fltwro carbons 

hydrocarbon olefin, such as cyclohexene or but-2-ene, to give the corre- 
sponding grem-difluorocyclopropane, often in high yield; 32 - 33 reaction of the 
difluorocarbene with cis- or with traws-but-2-ene is stereospecific, 33 in- 
dicating that the carbene is liberated in the singlet state, 34 e.g., 

F 2 C CF 2 + C=C *■ C C + OF 2 :CF 2 

A convenient but tedious laboratory preparation of perfluorocyclopropane 
involves photolysis of tetrafluoroethylene in the presence of ethylene 36 
(which does not compete successfully against the fluoro-olefin for the di- 
fluorocarbene intermediate, and acts as an inert diluent) or mercury i 36 

F 2 

, u.v. light, 180°/2 atm. C 2 F 4 / \ (67%) 

OT » ! °*« -oA^HStST^ 2:CF * *&— CT * 

The volatility of perfluorocyclopropane (b.p. —30°) enables gas-phase 
reactions of difluorocarbene to be studied under neutral, fairly mild con- 
ditions (see the Index for the locations of information on other sources of 
difluorocarbene, and also reference 34). 



II. PERFLUORO-ALKENES.-ALKADIENES*, -CYCLOALKENES , 

AND-CYCLOALKADIENES 
A. Preparation 

Fluorocarbon olefins cannot be made directly from hydrocarbon olefins 
by treatment with fluorine or a reactive high- valency metal fluoride (e.g., 
CoF s ), or by electrochemical fluorination, since these methods lead to 
addition of fluorine across C=C bonds; and only one example has been 
reported of simple and complete replacement by fluorine of chlorine in a 
chlorocarbon olefin via a single-step exchange reaction : 37 

ClaC CC1 ,. , „,, „„„„ F 2 C CF 

I II anhydroaa KW, 200° a i 11 

„, I II in Jf-methyl-2-pyrroUdone * I 'I * 72 "' 

C1 2 C\ /CC1 F 2 C\ y-GS 

Cl 2 F 2 

The general principle involved in the synthesis of a fluorocarbon olefin 
is to prepare a saturated polyfluoro-compound containing a structural 
feature that will enable a C=C bond to be introduced in the last stage 

* Except perfluoroalka-l,2-dienes (perfiuoroallenes), which are discussed in a 
separate section beginning on p. 53. 

20 



Perfluoro-Alkanes, -Alkadienes, -Cycloalkenes, and Cycloalkadienes 

through an elimination reaction which must leave only fluorine attached 
to the carbon atoms. Three types of elimination reaction are employed: 
dehydrohalogenation (including dehydrofluorination) of hydrofluorocarbons 
and halogenohydrofluorocarbons, dehalogenation of fluorocarbon 1,2-di- 
halides, and decarboxylation of sodium salts of fluorocarbon carboxylic 
acids. The first two types of reaction are used widely to prepare olefins, but 
the last one (olefin formation via decarboxylative dehydrohalogenation of 
a /S-halogeno-acid) is not often encountered in hydrocarbon chemistry. The 
use of these reactions will be illustrated by reference to standard methods 
of synthesis of some common fluorocarbon olefins. 

Tetrafluoroethylene, the simplest fluorocarbon olefin, is made commer- 
cially by pyrolytic dehydrochlorination of chlorodifluoromethane, 38 which 
is obtained from chloroform by a Swarts-type reaction : , 

CHC1 3 HF ' SbC ' 5 > CHF 2 C1 



eo. 700° 
platinum tube 



2CHF.C1 : ->• CF 2 :CF,+2HC1 

™ r\la 4-fmi-m tuna * * 



This pyrolysis involves a-elimination of hydrogen chloride from the halo- 
genomethane and dimerisation of difluorocarbene thus liberated. 39 Tetra- 
fluoroethylene is used commercially for the production of polytetrafluoro- 
ethylene, and this resin forms a convenient laboratory source of its monomer 
since it depolymerizes when pyrolysed at low pressure : *° 

600°/5mraHg 
[-CF 2 .CF 2 -]„ gteeltnb /> »CF 2 :CF 2 (97%) 

Pyrolysis of polytetrafluoroethylene also provides a convenient route to 
perfluoroisobutene : when it is heated at 450° under atmospheric pressure 
and the gas evolved is passed through a stainless steel tube held at 700°, 
the product is a mixture of perfluoroisobutene, perfluorobut-2-ene, and per- 
fluoropropene : 41 

450° 700° 
[— CF 2 .CF 2 — ]„ v gas >- (CF 3 ) 2 C:CF 2 (50%), CF 3 .CF:CF.CF,(9%), 

CF,.CFsCF 2 (30%) 

An alternative simple route to perfluoroisobutene (95% yield) involves 
passage of perfluoropropene through a nickel tube heated to 750°. 48 

Perfluoropropene is made commercially by low-pressure pyrolysis of 
tetrafluoroethylene : m 

CF 2 :CF 2 -^ °° /mmmHg > CF s .CF:CF 2 (8 2 o/o) 

but in the laboratory it is prepared by thermal decomposition of dry 
sodium perfluoro-n-butyrate in a Pyrex flask : 44 



200-250° 
CF 8 .CF 2 .CF 2 .C0 2 Na >- OF 3 .CF:CF 2 (97%)+C0 2 +NaF 



21 



Aliphatic Fluorocarbom 

The perfluoro-n-butyric acid required can be purchased; it is made com- 
mercially by electrochemical fluorination of n-butyryl fluoride followed by 
hydrolysis of the resultant perfluoro-n-butyryl fluoride : 

n .C 8 H,.COF "f^T 1 * 1 * n-CW-OOP ~^* n-Q t P,.0O t H 

8 ' fluorination °^ 7 

Pyrolysis of anhydrous sodium salts of perfluoroalkanecarboxylic acids 
provides an excellent general route to fluorocarbon terminal olefins: 44 

CF s .[CFJ».CF s .CP 2 .C0 2 Na -^* CF 8 .[CF 2 ]„.CF:CF 2 (60-90%) + CO 2 +NaF 
which has been used to obtain perfluorobuta-l,3-diene: 46 

100-460°/10-» mm Hg 



NaOjC.CCFjj-COiiNa 



CF a :CF.CF:CF 2 (37%) + CF 2 :CF.CF 2 .CF 2 .COF(21%) 
\ NaOH aq . 

CF 2 :CF.CF 2 .CF 2 .COijNa 



100-450 o /10- a mm Hr* 
(61%) 



The depth of the dry salt bed must be kept small to avoid prolonged contact 
between the olefin and the hot sodium fluoride produced concomitantly, 
since such contact can lead to isomerisation of perfluoro-a-olefins to more 
stable internal olefins (cf. p. 34). Owing to the closeness of their boiling 
points, the separation of isomeric terminal and internal perfluoro-olefins is 
a tedious business; thus preparation of perfluoro-a-olefins by flow pyrolysis 
of perfluoroalkanecarboxylic acids has been advocated 46 since the low 
catalytic activity of hydrogen fluoride minimises the possibility of iso- 
merisation. 

K F .CF,.CF,.C0 2 H J 0(K6 f°° > R F .CF:CF 2 +C0 2 +HF 
222 Monel tube 

Experimentally, this method is not as convenient as pyrolysis of sodium 
perfluoroalkanecarboxylates, which mechanistically is believed to involve 
^-elimination of fluorine as fluoride ion from a carbanion produced by loss 
of carbon dioxide from the carboxylate ion, 45,47 e.g., 

CFs.CF 2 .CF 2 .C02-Na+ >- CF3.CFa.GF2.COa- > CF3.CF2.CF2n 

F^-CF-^CF^ > F-+CF3— CF=CF 2 

CF 3 

The counterpart of the last stage in hydrocarbon chemistry is olefin for- 
mation by loss of a proton from a carbonium ion : 

H-^CH^CH-CHs ► H++CH 2 =CH-CH 3 

22 



Perfluoro-Alkenes, -Alkadienes, -Cycloalkenes, and -Cycloalkadienes 

A reaction often used to prepare fluoro-olefins in general is dehalogenation 
of appropriate 1,2-dihalides with zinc dust suspended in a polar solvent 
such as ethanol : 17 

— CFX.CFY— HZn ethan °' > — CF:CF— +ZnXY 

(X, Y usually = CI, Br, or I; a few cases are known where 
X = F, Y = CI, Br, or I) 

This type of elimination reaction is used, for example, in the last stages 
in the syntheses of perfluorobut-2-ene, perfluorocyclobutene, perfluorobuta- 
1,3-diene, and perfluorocyclopentadiene from commercially-available chloro- 
carbons : 

Perflitorobut-2-ene w 

CCVCCl.CCl:CCl a s ™>^> c \ CF 3 .CC1:CC1.CF !> (85%) Vb0 "™ 

CF 3 .CFC1.CFC1.CF„(26%) Zn ' ethanol > CF 3 .CF:CF.CF.(100%) 

AOAb 

Perfluorocyclobutene 4 ' 9 

CCI3.CCI3 Hi»^ cp.ca.cara, Zn ' eth t ano1 > cf 2 :Cfci ( 9o%) 200 ° 



180 heat * autoclave 

CF 2 — CFC1 „, t . , CF t -CF 

I I «/ 1 Zn > ethanol 1 m 

' I I (80%) t^~* I II ( 100% > 

CF 2 — CFC1 CF 2 — CF 

Perfluorobuta-1, 3-diene?° 

CF a :CFCl ™ in methylene chlor^ ^,01.0^76%) Hg, n.v. light f 

CF 2 C1.CFC1.CFC1.CF 2 C1(82%) Zn - etnapo1 , C F 2 :CF.CF:CF 2 (98%) 

neat 

Perfluorocyclopentadiene? 1 

C1C CCJ o^e 9nn = FC1C CFC1 J . FC CF 

II Co:F„200° I I . ... Zn, dioxan V u 

I' 1 * (33%) heat ? (44%) 

ClC Xc /CCl FaC Xc /CFCl he8t FC X /CF 

Cl 2 F 2 F 2 

Although the dehydrohalogenation reaction is used fairly frequently to 
prepare partially-fluorinated olefins, 17 it finds only limited use in synthesis 
of fluorocarbon olefins. Solid potassium hydroxide or a strong aqueous 
solution of this base is generally used as dehydrohalogenating agent when 

23 



Aliphatic Fluorocarbons 

highly-fhiorinated olefins are involved; alcoholic potash is not employed, 
since, as explained later, these olefins react with alcohols in the presence 
of base to yield ethers. A typical dehydrohalogenation forms the last stage 
in each of the syntheses shown below. 

Perftuorocyclo-hexene* 2 and -hezadieneip 3 




controlled fluorination 

3 

with C0F3 at ca. 150° 



F-.CK' ^CHF FjO^ X CHF 



hot 18JV-KOH aq. 



strongly-basic 
anion exchange resin, 50' 



+ 



F 2 C\ /CF 2 



FoCK X CF 



F a C\ c /CF 

F 2 
(82-97%) 



F«Ck 



,,/GF* 



+ other polyfluoro- 
cyolohexanes 



HF 



hot 182T-KOH aq. 



F 



F 2 C 



'V 



CF FC 



F 2 



F F 2 

(20%) (40% 



CF 

II 
|l 

•CF 



Perfluorobuta- 1 , 3 -diene 54 
CF 2 :CHF+IC1 - 



, powdered KOH in 
> CF 2 C1.CHFI(72%) — -■ > 



CF 2 :CFI(76%) 



mineral oil, 95° 
CF»:CB3t 



u.v. light 



> CF 2 :CF.CHF.CF a I(39%) 



powdered KOH in 
mineral oil, 95° 



CF a :CF.CF:CF a (67%) 

B. Physical Properties 

Fluorocarbon olefins are colourless, volatile compounds with high den- 
sities and refractive indices; 17 their boiling points lie close to those of the 
corresponding saturated fluorocarbons (see Table 2.4). Tetrafluoroethylene, 
perfluoropropene, and perfluoroisobutene are toxic, 55 - 86 and in view of this 
fact and the absence of information on the physiological properties of 
other members of their class, it is advisable to manipulate all fluorocarbon 
olefins with great caution. Perfluoroisobutene is particularly dangerous 
since it is more toxic than phosgene; thus care must be taken not to over- 
heat saturated fluorocarbons (e.g., polytetrafluoroethylene) inadvertently, 
or to manipulate or to release fluorocarbon pyrolysis products without 



24 



Perfluoro-Alhenea, -Alkadienes, -Cycloalhenes, and ■Cycloalkadienes 

Table 2.4. Boiling Points (°C) of Some Fluorocarbon Olefins and of their Saturated 

Counterparts 



CF2^CF2 


-76 


C 2 F e 


-78 


CF 2 — CF 


CF§ CFg 


CF 3 .CF:CF 2 


-29 


C 3 F 8 


-38 


1 1 5 
CF 2 — CF 


1 1 -« 

CFg CFg 


CF 3 .CF 2 .CF:CF 2 


1 



n-C 4 F 10 


-1 






CF S .CF:CF.CF 3 


1 1 2 3-5 
F 2 C\ c /CF 


1 1 22 


(CF 3 ) 2 C:CF 2 


7 






F 2 C\ /CF 2 


CF 3 .[CF 2 ] 2 .CF:CF 2 


30 


n-C 5 F, 2 


29 


F 2 

F 2 

FaC^ X CF 

1 II «3 
F *C\ C /-CF 

Fa 


F 2 

Fa 

FaC^ V CF 2 

1 1 53 
F 2 C\^ ^CF 2 
C/ 

F 2 



proper precaution, because this olefin can be formed in thermal reactions 
of fluorocarbons (e.g., see p. 21). As a general rule, inhalation and in- 
gestation of all fluorinated compounds, both organic and inorganic, should 
be avoided, since they cover a broad spectrum of toxicity and include 
compounds of extraordinary hazard; 66 even saturated fluorocarbons, which 
are non-toxic when pure, may contain toxic impurities as prepared in the 
laboratory. The fluorinated refrigerants and aerosol propellants (e.g., 
CF 2 C1 2 , CFC1 S ) available commercially are non-toxic. 86 A hazard that 
should be clearly noted is that tetrafluoroethylene under pressure can ex- 
plode violently, giving carbon and carbon tetrafluoride. A tetrafluoro- 
ethylene explosion is usually thermally initiated, and since the most likely 
accidental initiator is a hot-spot caused by the onset of uncontrolled free- 
radical homopolymerization, the olefin should be inhibited with a terpene 
when stored under pressure. 105 

Evidence exists 5 ' which indicates that the double bond in a perfluoro- 
olefin may be weaker than that in the corresponding hydrocarbon. For 
example, addition reactions of perfluoro- olefins (including homopolymeri- 
zation of tetrafluoroethylene) are more exothermic than the corresponding 
hydrocarbon reactions, and mercury-photosensitized decomposition of 
tetrafluoroethylene gives perfluorocyclopropane and polytetrafluoroethylene 
(via formation of :CF 2 ; c/. p. 20), whereas with ethylene no cleavage of 
the C : C bond occurs. The results of a study of the thermal trans-cis iso- 
merisation of perfluorobut-2-ene have been interpreted as direct evidence 
for the destabilisation of a C:C bond by fluorine substituents, and the 
difference (6-8 kcal/mole) in the activation energies of isomerisation of 
perfluorobut-2-ene and but-2-ene has been suggested to represent a measure 
of the lowering of the ^-uncoupling energy owing to fluorine substitution, 

3 25 



Aliphatic Fhtorocarbona 

attributed to the contribution of structures such as (A), which would tend 
to decrease the restriction to rotation about the C:C bond. 57 * 






F 



F»C 



/ 



\, 



CF, 



(A) 



F 
CF» 



C. Reactions 5 * 

1. With Nucleophilic Reagents. In contrast to hydrocarbon olefins, fluoro- 
carbon olefins characteristically undergo addition reactions with nucleo- 
philes and resist electrophilic attack. This is a consequence of electron 
withdrawal by the strongly electronegative fluorine atoms, which causes 
the electron density at the olefinic carbon atoms to be considerably de- 
pleted. 

Three types of product can arise through nucleophilic attack on a per- 
fluoro-olefin, as illustrated below for an a-olefin. In general, straightforward 
addition of the elements of Nu— X across the C : C bond predominates in 
the case of a low-molecular-weight a-olefin [reaction type (i)], but with 
internal, branched, and cyclic olefins there is a definite tendency towards 
the formation of unsaturated products [reactions (ii), mainly, and (iii)]. 



(i) 






Rf 



-» Nu.CF 2 .C<f 

V 



■Rf 



/ 

[x ^j> Nu.CF 2 .CX 



■CF2 # Rp 



(ii) / 

■* Nu.CF:C 



Rf 
'CFg.Rp 



— F 



\- 



Rp 



Rf 



(iii) /r 

—=Z* Nu.CF 2 .C 

x Rp 

(Nu" = nuoleophile derived from NuX ; Rp = F or perfluoroalkyl) 

Tetrafluoroethylene combines readily with alcohols, phenols, thiols, thio- 
phenols, and ketoximes in the presence of their sodium salts to yield 1 : 1 
adducts (see Fig. 2.5), 89 and it is thought that these reactions proceed in 
stepwise fashion as follows : 

Nu-+CF 2 :CF 2 ^e-determlning > Nu ^ ^_ _NuH > Nu-CTVCHFi+Nu - 
stop 
(Nu- = C 2 H 5 .0-, C 6 H b .O", C 6 H 5 .S-, etc.) 

Evidence for the formation of carbanions from tetrafluoroethylene and 
sodium alkoxides has been obtained by carrying out reactions in the pres- 
ence of esters to trap these intermediates; for example, when tetrafluoro- 



26 



Perfhtoro-Alkene8, -Alkadienes, -Cycloalkenes, and -Cydoatkadienes 




I 
1 






I* 

I 

1 1 

•fs 

a 

■a'J 

XI 

^*. 

11 



li 



* 
■^ 



•CM 

o 
S 



27 



Aliphatic Fluorocarbons 

ethylene is passed into a solution of sodium methoxide and dimethyl car- 
bonate in tetrahydrofuran at 20°, methyl ^-methoxytetrafluoropropio- 
nate (II) and l,5-dimethoxyoctafluoropentan-3-one (III) are obtained in 
74% and 12% yield, respectively. 60 This type of result can be rationalized 
mechanistically as follows : 

CH 3 .0-+CF 2 :CF 2 > CH 3 .O.CF a .CF a - 

'I J ' * 



n* 



r>* 



CH,.O.CF,.CF»- ^G-O.CH 3 ^=± CH3.0.CF 2 .CF 2 -C-O.CH 3 - 



O.CHa O.CH 3 

43H3.0.CF2.CF2.C02.CH3+CH3. e'- 
en) 



n*' i-^ 



CH,.O.CF,.CF,- (3-O.CH 3 ^=± Ctf3.aCF2.CF2-C-O.CH3 ^==± 
GF 2 .GF 2 .O.GH 3 CF2.GF2.O.CH3 

CH 3 . 0.GFa.GFg. GO. GF 2 .GF 2 .0. CH3 + CH3.O- 
(III) 

Secondary amines and ^-substituted amides also react with tetrafluoro- 
ethylene in the presence of their alkali-metal salts to yield 1 : 1 adducts, 
but strongly-basic aliphatic secondary amines require the assistance of no 
basic catalyst (see Fig. 2.5) . 59 Primary amines combine with tetrafluoro- 
ethylene to give a.a-difluoroacetamidines (IV), presumably by a series of 
addition-elimination reactions; ^-substituted a,a-difluoroacetamides are 
obtained in almost quantitative yields when these acetamidines are hydro- 
lysed with warm aqueous sodium carbonate solution : 59 

CF 2 :CF 2 +R.NH 2 > CHF 2 .CF 2 .NH.R ^^* CHF 2 .CF:N.R V 

CHF a .CF(NH.B) 2 ^^ CHF 2 .C(:N.R).NH.R -~^£> CHF 2 .CO.NH.B 

(IV) 
(R = alkyl, cycloalkyl, or aryl) 

The tertiary amines containing a tetrafluoroethyl group that are formed 
in addition reactions between tetrafluoroethylene and secondary amines 
yield ^iV-disubstituted a,a-difluoroacetamides on treatment with water, 59 
e.g., 

< 60° H2O 

CF 2 :CF 2 + (C 2 H 5 ) 2 NH ► CHF 2 .CF 2 .N(C 2 H 5 ) 3 (77%) ^ ^ ^ ^ KQH ^ > 



[2HF + CHF 2 .C(OH) 2 .N(C 2 H 5 ) 2 ] -^> CHF 2 .CO.N(C 2 H 5 ) 2 (~ 100%) 



28 



Perflvoro-Alkenes, -Alkadienes, -Cycloalkenes, and -Cycloalkadienes 

Facile nucleophilic displacement of the fluorine atoms of a CF 2 group 
adjacent to a nitrogen atom carrying alkyl, aryl, or acyl substituents is a 
general phenomenon, and the amine CHF 2 .CF 2 .N(C 2 H 5 ) 2 , for example, 
not only evolves fumes of hydrogen fluoride in moist air, but reacts with 
hydrogen cyanide and hydrogen sulphide at low temperature as follows : 59 

HCN , 
> CHF 2 .C(CN) 2 .N(C 2 H 5 ) 2 



CHF 2 .CF 2 .N(C 8 H 6 ) 2 - x 

— ^* CHF 2 .CS.N(C 2 H5) 2 

Possible reaction mechanisms have been presented 61 for these and related 
reactions involving the conversion of alcohols and carboxylic acids into 
alkyl and acyl fluorides, respectively, by amines of the above type. 

Anhydrous ammonia reacts exothermically with tetrafluoroethylene to 
yield tris(difluoromethyl)-l,3,5-triazine: M 



, — 2HF 
CF 2 :CF 2 +NH 3 > [CHFj.CFa.5IHj >■ 



CHF, 



trimerizes N N „. . 

CHFj.CN > n | (100%) 



-> N 

FaHC^^N^^CHFa 

Difluorostilbene (VIII) is obtained in 50% yield when tetrafluoroethylene 
is passed into an ethereal solution of phenyl-litbium at —80° and the 
mixture is allowed to warm up to room temperature. 83 Again a series of 
addition-elimination reactions would account for this result : 

C,H 5 Li+CF 2 :CF 2 > C 6 H 5 .CF 2 .CF 2 Li —^> C^.CF-.O^^^X 

(V) (VI) 

-L1F 



(VII) (VIII) 

Some support for the above reaction scheme is provided by the ready de- 
composition of perfluoroalkyl-lithium compounds containing the elements 
of structure present in the postulated intermediates (V) and (VII) (see 
p. 103). If (perfluorovihyl)benzene (VI) is required, phenyl lithium must 
be added to an excess of tetrafluoroethylene in ether at — 80°. Details of 
reactions between tetrafluoroethylene and some other organic derivatives 
of alkali metals are given in Fig. 2.6. 

Perfluorocyclobutene is one of the few perfluoro-olefins which have been 
treated with a Grignard reagent; 64 as in reactions between tetrafluoro- 

29 



Aliphatic Fluorocarbons 



30 




73 
8 



s 

e 



*5 



I 



05 

to 
eq 

6 

[S 



Perfluoro-Alkmea, -Alkadienes, -Cycloalkenes, and -Cycloalkadienes 



ethylene and phenyl-lithium, etc., the overall reaction is replacement of 
vinylic fluorine by an alkyl or aryl group, e.g., 



CF 2 — CF 



CFjr- CF 



0°, ether 
+ C*H 5 .MgBr >- 



CF a -C<. 
CF 2 — O 



C2H5 
F 
,/MgBr 



\> 



CF* — C C2H.5 



CF,— CF 



(75%) + MgBrF 



(The instability of perfluoroalkyl Grignard reagents is discussed on p. 107.) 
Perfluorocyclobutene is also one of the few fluorocarbon olefins whos© 
reactions with tertiary amines have been investigated; 85 the initial product 
is a quaternary fluoride, which reacts readily with water to give a trialkyl- 
(3,3-difluoro-2,4-dioxocyclobutyl)ammonium betaine (or a corresponding 
pyridinium betaine if the amine employed is pyridine or one of its deriva- 
tives), e.g., 



(CH,)sN CF=CF 
CF*t CFj 



.1 



(CH,)aN— C— 'QjF 



CFj CF2 



+ F" 
(CH 3 ),N— C=CF 

CF2 — CFg 



F" 



(CH^-C^CF OH 2 hj , 
CF— CF. 

(I ■ 



+ F~ / 
(CH,),N— C— C<. 

II I VI 



•OH 



CF— CF 2 



(CH,)iN-Oj-C^O 
CF— CF» 



(CHs)^— C=C-0- _ 2HF 

*\ I I * 

+ X3- CF S 
H 2 (K 



H.O 



(CH,),N— C=C— O" 
0=C— CF 2 



(CH,),N— C— C=0 
"O— C— CF 4 



(The instability of compounds containing a > CF.OH group is referred to 
on p. 165.) 



31 



Aliphatic Flw>rocarbon8 

Other fhiorocarbon olefins react with micleophilic reagents in similar 
fashion to tetrafluoroethylene, but it appears that as the attacking species 
becomes more basic and the stability of the presumptive intermediate 
fluorocarbanion increases (order of stability : tertiary > secondary > pri- 
mary), the tendency for overall substitution of vinylic fluorine increases at 
the expense of simple addition across the olefmic bond, e.g., 



CF 3 .CF:CF 2 C ' H »; 0H - K °H CF 3 .CHF.CF 2 .O.C 2 H 5 <85%) 



(CF 3 ) 2 C:CF 2 C ' H " 0H ' 20 \ (CF 3 ) 2 CH.CF 2 .O.C 2 H 5 (59%) + (CF 3 ) a C:CF.O.C 2 H 5 (6%) 

(IX) (X) 



(CF 3 , 2 C=CF 2 g*W*~, 



(CF 3 ) 2 CH.CF 2 .O.CH(CH 3 ) 2 (35%) + (CF 3 ) 2 C:CF.O.CH(CH 3 ) 2 (26%) 
(IX) (X) 



Treatment of ethers of type (IX) with an excess of the hot parent alkanols 
does not dehydrofluorinate them to the corresponding unsaturated com- 
pounds (X), so it seems that the latter must be formed via elimination of 
fluorine as fluoride ion from intermediate carbanions, 86 e.g., 



(GF 3 ) 2 G— GF.O.G a H 5 > (CFs)«C:CF'.O.C»H« + F- 

F 2 



If fluorocarbanions of the type CF 3 .CF.CF 2 .O.R and (CF 3 ) 2 C.CF a .0.R 
[R = CsjH 5 , (CH 3 ) 2 CH] are intermediates in the above reactions, the pro- 
duct distributions could be accounted for simply by arguing that the latter 
has greater opportunity for resonance stabilization involving halogen hyper- 
conjugation (as annexed) than the former* and is therefore the weaker 
base and less able to stabilize itself by abstracting a proton from an alcohol 
R.OH, and by taking into account the basicity of the anion R.O~, since 
this determines the ease of abstraction of a proton from R.OH [(CH 3 ) 2 CH.O - 
is a stronger base than C 2 H 5 .0~]. 

* See p. 83 for a discussion of this effect. 
32 



Perfluoro-Alkenes, -Alkadienes, -Cycloalkenes, and -Cycloalkadienes 

F 3 C\_ F"F 2 C. F 3 C\ F' 

>C— CF 2 .Nu «-*■ >C— CF 2 .Nu •<->■ ~>C=CF-Nu 

F 8 <r FaC^ F3CK 



The orientations of products in reactions between perfhioropropene and 
perfluoroisobutene and nucleopbilic reagents are those expected from an 
appraisal (see p. 83) of the relative stabilities of the possible carbanionic 
intermediates involved; for example, the development of a negative charge 
on C2 of perfhioropropene to give CF 3 .CF.CF 2 .Nu would be predicted to 
be tolerated more than on C 1 to give CF 3 .CF(Nu).CF 2 . 

As might be expected from the above considerations, perfluoroisobutene is 
more susceptible to nucleopbilic attack than either of its two isomers or 
perfhioropropene; even in neutral or weakly acidic media it reacts with 
methanol and ethanol at room temperature to yield ethers, 6 * and it is 
slowly attacked by water at room temperature (see p. 97). 



R F .CF 2 .CHF.CF 3 R P .CF 2 .CF(CF 3 ).CO.R* +F" 

\ R*..C\ F / 

\ / 

R F .CF 2 .CF:CF a +F- ^=^ Rj,.CF 2 .CF.CF 3 ^=i F" +R P .CF:CF.CF 3 



R P .CF 2 .CF(CF 3 ).CF:CF.CF 2 .R F +F- 




6- <5 + 
Ef.CFi.CF:CF, 



a- a+ 

-= _ ~ (i)BF.CF 2 .CF:CF, 
R F .CF 2 .CF(CF 3 ).CF 2 .UF.CF 2 .R F — — _ F _ — -> etc. 

(R p = F or perfluoroalkyl) 



From the viewpoint of synthesis, a most important nucleophilic reaction 
of a fluorocarbon olefin is combination with a fluoride ion to yield a per- 
fluorocarbanion. Once generated, a perfluorocarbanion will undergo one or 
more, depending on the conditions, of three general reactions in order to 
stabilize itself: proton abstraction from a suitable donor, elimination of a 
fluoride ion to yield predominantly the most thermodynamically-stable 
fluorocarbon olefin possible, and attack on an electrophilic centre con- 
currently with or followed by the elimination of some negative species (see 
the annexed general scheme). Some examples of syntheses involving tran- 
sient perfluorocarbanions are given below; others are referred to elsewhere. 



33 



Aliphatic Fluorocarbons 



CF 8 .CF:CF 2 — " g M * M '> CF 3 .CHF.CF 3 (60%) (ref.67) 



CFa CF;CFa NaFindimettylformamide ^^.03^.^ (ref . 68) 



(CF 3 ) 2 C:CF.CF 2 .CF 3 , 
(CF 3 ) 2 CF. CF: C(CF 3 ) . CF(CF 3 ) 2 , 
(CF 3 ) 2 CF.CF 2 .C(CF S ):C(CF 3 ) 2 , 
(CF 3 ) 2 C:CF.CF(CF 5 ).CF(CF 3 ) 2 



CF 2 :CF 2+ C0 2 (1)KFto ;"" e - 150 °> C 2 F 5 .C0 2 H(75%) ( ref.69) 



(i) CsF in diglyme, 100° 
(ii) H a SO, 



CF a! CF 2 + (CF 3 ) 2 0:O -^ ^—r^ > (CF 3 ) 2 (C 2 F 5 )C.OH(86%) (ref.70) 



CF 2 — CF „ „ . . , t ., CF 2 — CF S 

I ■* :i CsF In aoetonitnle i i , . , 

I I + COF * 125-150° > i I (54)(ref.71) 

CF 2 — CF CF 2 — CF— COF 

KHF a in aceton.it rile _ 

CF 3 .CF:CF 2 +n-C 3 F 7 .COF -— — j= > n-C 3 F,.CO.CF(CF 3 ) 2 (60%) (ref.72) 



F 
/""V, CsF 

CF 3 .CF:CF 2 + N N -^ 



Rf Rf 



NN+NN+ NH (total>90%)(ref.73) 

Fi '-N^R P F ^Arf /A 
[Rf = (CF 3 ) 2 CF] 

Isomerisation of a perfluoroalk-1-ene to a perfluoroalk-2-ene can be 
achieved by treating it with a trialkylammonium fluoride in chloroform 
solution at room temperature 67 or by passing its vapour over a heated bed 
of alkali-metal, magnesium, or barium fluoride, e.g., 74 

(C«H«)*N + F - in chloroform, 25° _ 

CF 3 .[CF 2 ] 4 .CF : CF 2 orKF.lSO' * CF 3 .[CF 2 ] 3 .CF:CF.CF 3 (~ 100%) 

This type of isomerisation can occur when alkali-metal salts of perfluoro- 
alkanecarboxylic acids are pyrolysed, e.g., 76,76 

CF 3 .[CF 2 ] 8 .C0 2 K 27(m0n ™ Hg > CF 3 .CF 2 .CF:CF 2 (20%)+CF 3 .CF:CF.CF 3 (80%) 
34 



Perflvoro-Alkenes, -Alhadienes, -Cycloalkenes, and -Cychalkadienea 

Another very useful reaction of fluorocarbon olefins that involves nucleo- 
philie attack is oxidation with potassium permanganate; generally best 
carried out in acetone solution, 77 this reaction is often used in structure 
determination and to prepare dicarboxylic acids: 

CF^CF^.CI^CFa SMa0 '^ eU > a % C F 8 .[CF 2 ] 4 .CO a H(61%) 

^-f KMnQ.lu acetone OT r 0O^ 
CF 2 — OF CF,.COjH 

Permanganate oxidation of perfluoroisobutene yields hexafluoroacetone, 
which forms a stable hydrate (see p. 91) : tt 

(CF,) 2 C:CF 2 KM ° ' ""• > (CF s ) 2 C(OH) 2 P '°' > (CF 3 ) 2 0.0 

Epoxidation of fluorocarbon olefins is discussed later (see p. 162). 

2. With Electrophilic Reagents. Although it has been known for some 
time that several types of reagent that add across double bonds in hydro- 
carbon olefins by an electrophilic mechanism will combine with some 
fluorocarbon olefins under ionic conditions to yield 1:1 adducts, only 
recently has any definite evidence been found for the attack of an electro- 
phile on a fluorocarbon ^-electron system. This evidence mainly concerns 
attack on the double bond in perfluoropropene by the mercuric cation 
generated by dissolution of mercuric fluoride in anhydrous hydrogen 
fluoride, which strongly solvates fluoride ions to give (H B F B+1 )~ ions that 
apparently are too weakly nucleophilic to attack a fluorocarbon olefin: 

xrx» 

HgF a * Hg+++2(H„F„ +1 )- 

(HgF a forms a 0-5% solution in liquid HF at 11-9°) 

Thus perfluoropropene, which is stable to anhydrous hydrogen fluoride at 
temperatures up to 200° and to anhydrous hydrogen fluoride containing 
potassium fluoride at 125°, reacts with a mixture of mercuric fluoride and 
anhydrous hydrogen fluoride at 85° to yield bisheptafluoroisopropyl- 
mercury: 78 ' 79 

85° HP 

2CF 3 .CF:CF 2 +HgF 2 ^^» [(CF 5 ) 2 CF] 2 Hg(65-80%) 

This reaction, which occurs to only a minor extent in the absence of an- 
hydrous hydrogen fluoride, has been suggested to involve the formation 

\ 35 



Aliphatic Fluorocarbons 

of an intermediate cyclic mercurinhim ion, 79 as proposed for hydrocarbon- 
olefin mercuratdon reactions : 



CF 3 

FC +Hg + 

II 
F 2 C 



CF 3 

I 
FC. 

I 
Lf 2 C 



;=Hg 



+ CF 3 

J- I 
>■ FC— Hg + 

F— F 2 C 



CF 3 

I 
FC + Hg— CF(CF S ) 2 

II 
F,C 



CF 3 
I 
FC-. 



F 2 C 



>Hg— CF(CFs)2 



v- 



from (Hn^n+l)- 

CF» 



FC— Hg— CF(CF 3 ) 2 
F— F 2 C 

In the absence of mechanistic information other than knowledge of the 
manner in which the reagent attacks a hydrocarbon system, consideration 
must always be given to the possibility that an electrophilic reagent in 
the hydrocarbon sense will attack a fluorocarbon olefin by a nucleophilic 
or a four-centre mechanism. 

In general, fluorocarbon olefins resist attack by reagents that are classed 
as 'electrophilic' in hydrocarbon chemistry, especially halogen halides. Some 
typical reactions are shown below : 78 



CF 2 :CF 2 +N0 2 C1 > CF 2 C1.CF 2 .N0 2 <57%) 

26°, no light 
CF„.CF:CF 2 +HBr 



-> No reaction 



X AlBr " 60 ° ., CF 3 .CHF.CF 2 Br(80%) 
no light 



Br s> 25 



CF 3 .CF:CF.CF 3 



/ very slow reaction 
HBr, 280 



->• CF 9 .CFBr.CFBr.CF. 



■V No reaction 



CF 2 — CF Tri 1ano CF 2 — CFC1 CF 2 — CFC1 

| || I01, 18 ° > | ] (77%) + | | (10%) 

CF 2 -CF CF 2 — CFI CF 2 — CFC1 



FX^ X CF 



HX(X = Br or I), 250 



> No reaction 



F,C 



F, 



-CF 



36 



Perfluoro-Alkanes, -Alkadienes, -Cycloalkenes, ami -Cycloalkadienes 

Although aluminium halides catalyse the addition of hydrogen halides 
across C : C bonds in fluorocarbon olefins and also alkylation reactions with 
halogenomethanes, 58 undesirable side-reactions can occur through attack 
by aluminium halides on reactants, leading to fluorine substitution, and 
on products, leading to disproportionation. Regarding the former type of 
side -reaction, a mixture of cis- and iraws-perfluorobut-2-ene reacts with 
aluminium trichloride at 80° (but not at room temperature) to give a 
mixture of cis- and traws-2,3-dichlorohexafluorobut-2-ene, pentachloro- 
l,l,l-trifluorobut-2-ene, and perchlorobutadiene ; and treatment of per- 
fluorocyclobutene with an equimolar amount of aluminium trichloride in 
the presence of an excess of anhydrous hydrogen chloride at 0° gives per- 
chlorocyclobutene in 77% yield. 80 

3. With Free Radicals. In general, free-radical attack on the double bond 
of a fluorocarbon olefin proceeds smoothly and with an ease that can be 
correlated with the steric and polar factors involved. From results achieved 
with perfluoropropene, it appears that polar factors also influence the 
direction of addition of a free radical to the double bond of an unsymmetrical 
fluorocarbon olefin. 

Many free-radical addition reactions of tetrafluoroethylene have been 
studied, mainly with the synthesis of polyfluoroalkyl compounds as ob- 
jective (see Fig. 2.7). The telomerization reaction that occurs when a 
mixture of tetrafluoroethylene and trifluoroiodomethane is heated or irra- 
diated is particularly useful, since compounds of general formula 
CF 3 .[CF 2 .CF 2 ] M .I (where n = 0, 1, 2, 3, etc.) can be converted into many 
other fluorocarbon derivatives (see Ch. 4). The reaction can be controlled 
to give good yields of telomer iodides with n ~ 1 to 10, and the following 
mechanism has been proposed: 81 



220° 

CF,I ► CF..+I- 

3 or u.v. light 3 

Initiation 

CF 3 .+CF 2 :CF 2 > CF 3 .CF 2 .CF 2 - 

Propagation 

CF 3 .CF 2 .CF 2 . + (n-l)CF 2 :CF 2 >■ CF 3 .[CF 2 .CF 2 ]„. 



Chain transfer 



CT.:CF. 
CF 3 .CF 2 .CF a .+CF 3 I v CF 3 .CF 2 .CF 2 I+CF 3 . - > etc. 

CF 2 :CF. 
CF 3 .[CF 2 .CF 2 ]„.+CF 3 I * CF 3 .[CF 2 .CF 2 ]„I+CF 3 : !* etc. 



The strongly electrophilic trifluoromethyl radical adds more slowly to the 
electron-poor double bond in tetrafluoroethylene than the weakly nucleo- 
philic methyl radical ; the converse is true for the rates of addition of these 

37 



Aliphatic Flvorocarbons 




38 



Perfluoro-Alkenes, -Alkadienes, -Oyckxdkenes, and -Cycloalkadienea 

radicals to the electron-rich double bonds in ethylene, propene, and iso- 
butene. 82 

The direction of addition to the double bond in perfluoropropene during 
radical attack depends upon the nature of the attacking radical, as illustra- 
ted by the data given in Table 2.5. The results quoted there can be con- 
sidered in terms of two related factors: 83 (i) the reactivity of the attacking 



Table 2.6. Radical Addition to Perfluoropropene 





Attack (%) 


on starred 








carbon atom 








* 


* 




Radical attacking 


CF 8 


,CF:CF 2 


CF 8 .CF:CF 2 


Bef. 


(CH,) s Si. 


96 




4 


83 


(CH 8 ) 2 SiH- 


95 




5 


83 


CHj.S- 


91 




9 


84 


CF 3 . 


80 




20 


83 


CH 3 .SiHj,- 


76 




24 


83 


CF 8 .CH 2 .S- 


70 




30 


84 


PH,- 


66 




34 


83 


SiH,- 


60 




40 


83 


Br- 


60 




40 


85 


SF 5 . 


50 




50 


86 


CF 8 .S- 


45 




55 


84 



free radical (i.e., the extent to which derealization of the unpaired electron 
occurs), and the corresponding expected specificity of attack on perfluoro- 
propene; and (ii) the nucleophilic or electrophilic character of the attacking 
free radical taken in conjunction with the susceptibility towards nucleo- 
philic attack of perfluoropropene. 

Considering (i), one would expect that the more reactive a free radical 
is, the less it would discriminate between the two possible positions of 
attack on the double bond of an unsymmetrical olefin. Thus in the case 
of perfluoropropene, the more reactive the attacking radical the greater 
should be the extent of attack on the sp*-carbon atom of the CF S .GF group 
at the expense of attack on the carbon atom of the CF a group (the latter 
is sterically more favoured and leads to formation of the more stable inter- 
mediate radical). Examination of the data quoted in Table 2.5 shows that 
this is not observed ; the more reactive the radical is [e. g., (CH 3 ) 3 Si- > SiH 3 - ; 
CH 3 .S- > CF 3 .S-], the more discriminating it appears to become. 

As regards (ii), in any sequence of free radicals, the radical most reactive 
towards an olefin normally susceptible to nucleophilic attack should be the 

39 



Aliphatic Fluorocarbons 

one bearing the most powerful, or the most numerous, electron-releasing 
substituents, i.e., the most nucleophilic free radical, e.g., 

(CH 3 ) 3 Si.>(CH s ) 2 SiH.>CH 3 .SiH 2 .>SiH 3 .; 
or CH 3 .S.>CF 3 .CH 2 .S.>CF 3 .S., SF 6 - 

The reverse order should obtain for attack on an olefin normally susceptible 
to electrophilic attack. As well as determining the rate of addition, the 
polar character of the attacking free radical (R-) could be important in 
determining the site of attack, particularly when there is not much differ- 
ence in the relative stabilities of the two radicals that could be produced 
by attack on an unsymmetrical olefin; normally the difference in stability 
of these intermediate radicals is sufficient to determine the site of attack, 45 

e.g., 

R. + CF 2 :CFC1 > R.CF 2 .CFC1- rather than R.CFC1.CF 2 - 

R.+CH 2 :CF 2 > R.CH 2 .CF 2 - rather than R.CF 2 .CH 2 . 

R.+CF a :CF.CF 3 > R.CF 2 .CF.CF 3 rather than R.CF(CF 3 ).CF 2 . 

With perfluoropropene, however, the difference in stability between the 
intermediate radicals R.CF 2 .CF.CF 3 and R.CF(CF 3 ).CF 2 . does not appear 
to be great, so that the polar character of the attacking radical R- becomes 
important in determining the relative amounts of attack on the two olefinic 

carbon atoms. 

Perfluoropropene is highly sensitive to nucleophilic attack, which appears 
to occur exclusively on the CF 2 group (see p. 32), and the results of the 
radical addition reactions quoted in Table 2.5 clearly demonstrate that the 
more nucleophilic the attacking radical becomes, the more is attack orien- 
tated towards the CF 2 group. 

Apart from tetrafluoroethylene and perfluoropropene, not much is known 
about free radical attack on fluorocarbon olefins. Perfluoroisobutene is more 
resistent to photochemical bromination than perfluorobut-2-ene, and the 
latter reacts less readily with hydrogen bromide in the presence of u.v. light 
than perfluoropropene. In fact, photolysis of a mixture of perfluorobut-2-ene 
and hydrogen bromide yields a 60:40 mixture of the adduct CF 3 .CHF. 
.CFBr.CFg and the dibromide CF 3 .CFBr.CFBr.CF 3 , since photolysis of hy- 
drogen bromide occurs with liberation of free bromine which then undergoes 
slow photochemical reaction with the olefin. 76 Perfluorocyclobutene reacts 
photochemically with hydrogen bromide even more slowly than perfmoro- 
but-2-ene, with the result that the dibromide is the major product: 76 

OF.— CF CF 2 — CFBr CF 2 — CHF 

| * f HBr -" Y -' ight > -| | (49%) + | | (33%) 

CF 2 — CF CF 2 — CFBr CF 2 — CFBr 

40 



PLATE 3 



Polytetrafluoroethylene expansion 
joints, gaskets, plaited packing 
tape, and machined components 
for use in chemical plant. (Photo, 
by courtesy of Imperial Chemical 
Industries Ltd., Plastics Division, 
and Turner Brothers Asbestos Co. 
Ltd., Rochdale, Lanes.) 



Wires and cables insulated with 
polytetrafluoroethylene for use 
where high operating temperatures 
are encountered and very high 
frequency electrical insulation is 
required. (Photo, by courtesy of 
Imperial Chemical Industries Ltd., 
Plastics Division, and British 
Insulated Callender's Cables Ltd.) 



Valve holders and flying leads 
containing polytetrafluoroethy- 
lene. (Photo, by courtesy of Impe- 
rial Chemical Industries Ltd., Plas- 
tics Division, and The McMurdo 
Instrument Co., Ltd., Ashtead.) 





PLATE 4 

Bushes, thrust washers, and slides 
made of steel-backed sintered 
bronze, impregnated with poly- 
tetrafluoroethylene and lead. 
These components can operate 
without lubrication, and may be 
used where oil, grease, and other 
lubricants are undesirable, im- 
practicable, or unreliable. (Photo, 
by courtesy of Imperial Chemical 
Industries Ltd., Plastics Division, 
and The Glacier Metal Co. Ltd., 
Wembley, Middlesex.) 




Bakery rotary moulder fitted with 
polytetrafluoroethylene - covered 
rollers to prevent dough from 
sticking to them. {Photo, by cour- 
tesy of Imperial Chemical Indus- 
tries Ltd., Plastics Division, and 
Siemens Edison Swan Ltd.) 



Double-ended Crane Bellows Pump 
for use with highly corrosive 
chemicals, in which all parts that 
come into contact with the liquids 
or gases being metered are made 
from polytetrafluoroethylene. 
{Photo, by courtesy of Imperial 
Chemical Industries Ltd., Plastics 
Division, and Crane Packing Ltd., 
Slough.) 






Perflnoro-Alkenea, -Alkadienes, -Cycloalhenes, and -Cydoalkadienes 

Perfluorocyclohexene does not react at all with hydrogen bromide under 
the conditions used to effect the above result with perfluorocyclobutene, 
and it also shows a marked resistance to photochemical chlorination. 76 

The following addition reactions occur when mixtures of trifluoroiodo- 
methane and the olefins CF 8 .CF:CF.CF 3 , cyclo-C 4 F 6 , and cyclo-C„F 10 are 
exposed to u.v. light : 76 

CF 3 I+CF S .CF:CF.CF 8 °J^ 8ht > (CF„) 2 CF.CFI.CF 3 (79% ; unchanged olefin : 23 % ) 

CF 2 — CF „ . t CF 2 — CF.CF 3 

1 11 u.v. light 1 

CF S I + I ]| 25da > J I (47%; unchanged olefin: 30%) 

CF 2 — CF ayS CF 2 — CFI 

F 2 

F 2 (r X CF 
2 1 11 u.v. light, 60-65° 

CF aI + rr-. > 

1 11 45 days 

F 2 C\ /CF 
F 2 

F 2 F 2 

^i<y X CF.CF 3 FaC^ X CF.CF 3 

I I (68%*) + I I (11%*) 

F 2 C X XJFI F 2 C X /CF.CFs 

C C 

F s F 2 

* Based on 56 % of olefin consumed. 

(a) Free-Badical Addition, Polymerization of Fluorocarbon Olefins. During 
the past thirty years there has been an insistent and ever-increasing 
demand for polymers capable of service under extremes of conditions, 
particularly for military and allied purposes. Many of these demands have 
been met by polymers containing fluorine. Such polymers range from 
mobile liquids through greases to thermoplastics and elastomers, and are 
characterized by extreme chemical interness, great thermal stability, and 
excellent electrical properties. 87 These valuable characteristics offset then- 
current high cost. 

The present discussion is restricted to the fluorocarbon high-molecular- 
weight polymers polytetrafluoroethylene, [— CF 2 .CF 2 — ]„ (Teflon", Fluon 6 ),* 
polyhexafluoropropylene,** [— CF 2 .CF(CF 3 )— ]„, polyhexafluorobutadiene, 
[G*Ft] n > an d the copolymer of tetrafluoroethylene with hexafluoropropylene, 

* Trade names: a E. I. du Pont de Nemours & Co., U.S.A. 

b Imperial Chemical Industries, Ltd., England, 
c Minnesota Mining & Manufacturing Co., U.S.A. 
** The olefin CF 3 .CF:CF 2 is named hexafluoropropylene in this section in order 
to comply with current nomenclature used by polymer chemists. 

* 41 



Aliphatic Fluorocarbons 

[— (CF 2 .CF,) a ,.CF8.CF(CP^] n (Teflon 100 FEP a ), and to the copolymer of 
vinylidene fluoride withhexafluoropropylene,[— (CH 2 .CF 2 ) a ,.CF 2 .CF(CF 3 )— ]„ 
(Viton," Fluorel"). Only the first one and last two of these are produced on 
a commercial scale at the present time. 

(i) PoLyTBTKAinxROETHYiiENE. Polytetrafluoroethylene (PTFE) is the 
most important polymer containing fluorine and accounts for more than 
90 % of the estimated world consumption of such materials. It was developed 
on a technical scale during the Second World War, after which it was 
released for general industrial use and commercial exploitation. 

The rapid, exothermic polymerization of tetrafluoroethylene under 
moderate pressure in a stainless steel autoclave is initiated by aqueous 
ammonium persulphate, and must be carefully controlled to prevent ex- 
plosive decomposition of monomer to carbon and carbon tetrachloride. 
Depending on the detailed conditions, the highly- crystalline polymer 

FFFFFFFFFF 

I i I I I I I I I 1 
— C— C— C— C— 0— C— C— C— C— 0— 

I I I I I I I I I I 

FFFFFFFFFF 

may be obtained in either of two basic forms — as an ivory-white granular 
powder or as an aqueous dispersion. The polymer available commercially 
has a molecular weight lying within the range 500,000-5,000,000 and a 
specific gravity of 2- 1-2-3. 

PTFE has a working temperature range from 250° down to at least the 
temperature of liquid nitrogen (— 196°). Within this range it is virtually 
immune to chemical attack, being affected only by molten alkali metals 
and by fluorine and chlorine trifluoride at elevated temperatures. It 
is non-inflammable and will not dissolve in any known solvent, although 
it is swollen by a few fluorocarbon oils. PTFE is not wetted by water, 
does not absorb water, and has excellent weathering and ageing charac- 
teristics. 

PTFE is the best solid dielectric, having a very low power factor and per- 
mittivity, both of which are independent of frequency and temperature. 
It is also an extremely efficient electrical insulator, particularly so because 
it is non-tracking. 

The coefficients of static and dynamic friction of PTFE are numerically 
equal and are about the same as those of wet ice on wet ice ! In addition, 
it is the best of the non-stick materials and few substances will adhere to 
its surface. It has been suggested that the remarkably low coefficient of 
friction of PTFE is due to the smooth profile of the rod-like polymer 
molecules (see p. 4), which permits easy slip of one chain past an- 
other and easy gliding on the crystal planes that he parallel to the 
chain axes. 14 

42 



Perfltwro-Alkenes, -Alkadienes, -Cycloalkenes, and -Cydoalhadimee 

All these properties are being exploited in the chemical, electrical engi- 
neering, and allied industries. Some products containing PTFE are shown 
in Plates 3 and 4. 

Probably the main limitation of PTFE is that it cannot be converted 
into useful forms by rapid means; when it is heated above its melting 
point (327°) it becomes an amorphous transparent gel that is mechanically 
weak and will not flow without fracture, therefore it cannot be fabricated 
by the standard moulding and extrusion methods that are applied to normal 
thermoplastics. The fabrication techniques that have been developed— cold- 
forming followed by sintering at 350-400°— are analogous to those used 
in powder metallurgy. 

(h) TETBAPLUOBOETHYLENE-HEXAFLTJOBOPBOPYLENE COPOLYMER. 

Teflon 100 FEP (Fluorinated Ethylene Propylene) thermoplastic resin, 
probably a random copolymer containing mainly tetrafluoroethylene re- 
sidues 

F F F F F CF 3 F F F F F F F CF 3 

I I I I I I I I I I I I I | 

-c-c-c-c-c-o c-c— c-c-c— c— c-c- 

! I I I I I I I I I 1 I I I 

FFFFFF FFFFFFFF 

equals PTFE in chemical inertness and insolubility, but has a lower melting 
point (ca. 290°) and somewhat inferior (though still excellent) electrical 
properties. However, it has the advantage of being a true thermoplastic 
that can be fabricated in conventional extrusion and injection moulding 
equipment. Quantity production of this copolymer, for which a wide range 
of uses is claimed, began in the United States towards the end of 1959. 

(iii) Polyhexafltjobopbopylene. In striking contrast to tetrafluoro- 
ethylene, hexafluoropropylene is difficult to homopolymerize under free- 
radical conditions. Only recently has high-molecular-weight polyhexafluoro- 
propylene been obtained, and its preparation involves the use of high 
temperatures and pressures, 88 e.g., 

nCF 3 .CF:CF 8 < CT »-S>*H* (see p. 177) 

3 2 225°/3000 atm L ^2 ^l^aJ J» 

Like PTFE, polyhexafluoropropylene is a white, dense solid, which has 
great chemical resistance and an excellent set of electrical properties; but 
unlike PTFE, it is amorphous when prepared as above, soluble in fluoro- 
carbons, and a normal thermoplastic. It softens at 225-250°, and when 
subjected to vacuum pyrolysis at temperatures above 275° depolymerizes 
to hexafluoropropylene and traces of tetrafluoroethylene and perfluoro- 
isobutene. 

Interestingly, both tetrafluoroethylene and hexafluoropropylene have 
been claimed recently to undergo slow homopolymerization to high-molec- 

43 



Aliphatic Fluorocarbons 

ular-weight crystalline polymers in the presence of Ziegler-Natta type 
catalysts. 89 It is thought that polyhexafluoropropylene, m.p. 110-120°, 
prepared in this manner will probably be found to have an isotactic struc- 
ture : 89 



[(CH,) a CH.O].Tl/t(0H,),CB.CH,I,Ai^ 
82 methylene chloride; 30»/15days 



F 


F 3 


F 3 F 3 
.G F .G 


2%) 


F 2 


Fa 


F 2 







(iv) Polyhexapluobobutadiene. Hexafluorobuta-l,3-diene is more diffi- 
cult to homopolymerize to a high-molecular-weight polymer than hexa- 
fluoropropylene; it is claimed to yield a rubbery solid when heated with 
peroxide initiators at extremely high pressures, e.g., 16,000 atm. 90 

(v) ViNYLIDENE SXTJOBIDE-HEXAFLUOEOPEOPYLENE COPOLYMEE. 

Several fluoro-elastomers were developed during the 1950's for use in high- 
speed aicraft ; the most important of these is the copolymer of vinylidene 
fluoride with hexafluoropropylene known as Viton and Fluorel, which has 
no equal in its resistance to fuels, oils, and solvents at temperatures above 
200°. Commercial Viton A contains approximately 60% vinylidene fluoride 
and 40% hexafluoropropylene by weight and is prepared by free-radical 
copolymerization of these monomers under the influence of aqueous am- 
monium persulphate at 100° : 91 

(NH 4 )aS a 8 aq., NaHSOj aq. 
:mCH 2: CF 2 +»CF 3 .CF:CF 2 -*— q> ^ 



100°/64 atm 

[— (CH 2 .CF 8 ) !t .CF iS .CF(CF,)— ]„ 

The elastomeric character of this copolymer is probably due to the presence 
of the methylene groups, which introduce flexibility into the backbone, 
and also to the presence of the pendent bulky trifluoromethyl groups, 
which reduce the tendency for the polymer to crystallize. The methylene 
groups also enable cross-linking of the copolymer to be achieved by the 
use of peroxides, /8-radiation, or polyfunctional amines. Cross-Unking by 
means of peroxides or ^-radiation is presumed to occur by hydrogen ab- 
straction and combination of the polymer radicals thus formed: 

— CH J .CF 2 .CF 2 .CF(CF 3 ).CH 2 .CF 2 — 5l2li _CH 2 .CF 2 .CF 2 .CF(CF 3 ).CH.CF 2 — 



— CH 2 .CF 2 .CF 2 .CF(CF S ).CH.CF 2 
— CH 2 .CF 2 .CF 2 .CF(CF 3 ).CH.CF 2 

44 



Perfluoro-Alkenea, -Alhodienea, -CycUxdkenes, and -Cycloalkadienes 

Cross-linking through reaction with polyfunctional amines, the basis of the 
preferred industrial procedure, involves either a base-catalysed elimination 
of hydrogen fluoride from the polymer followed by addition of the amine 
across the double bonds thus formed, or an $#2 reaction with displacement 
of fluorine — or a combination of both these processes. 

4. Hydrogenation. Saturation of the C:C bond in a fluorocarbon olefin 
with hydrogen can be effected smoothly and efficiently at moderate temper- 
atures in the presence of a ca. 1 % palladium-alumina catalyst, e.g., 92 



CFi,.CF:CF 2 - **** a "'°> CF S .CHF.CHF S (96%) 

Do— olr 



< ? F *~ 9 F H„Pd-AI,0, 



cf,— c<r 



CF 2 — CF 



55-60° 



CF 2 — a/' 



'••*■ (86% + 7% of the 
■gr trans-isomer) 



■F 



The reactions are exothermic and, owing to the high volatilities of fluoro- 
carbon olefins, can conveniently be carried out by passing the vapour of 
the olefin mixed with an excess of hydrogen through a glass tube packed 
with the catalyst. Trihydro-compounds may be obtained as by-products, 
and are believed to arise by hydrogenation of olefins formed by dehydro- 
fluorination of the dihydro-compounds formed initially, e.g., 98 



F 2 C- 

i 

F,C 



9 F Hj.Pd— A1,0, F 2 C 



V 



•H 



\n/' 



CF 



>90° 



FsP^c^Cx 



A1»0»,J 

(90%) (-HF) V 



-CH 



'F 



FiC- 
F 2 C Xc x CF 

Hi Pd— AM)* 
F 2 C CH< 



(10%) 



F 2 C\ /CHF 

F 2 



Replacement of vinylic fluorine in a fluorocarbon olefin by hydrogen can 
be effected with lithium aluminium hydride or sodium borohydride, which 
act as sources of hydride ion, 94 e.g., 



45 



Aliphatic Fluorocarbons 



F 2 




F 


2 






F 2 


F 2 


F 2 C'"' 




LiAlH 4 


F 2 CT ^CHF 

1 1 - 


-F- 


F a C' / 


II + 
C /CF 


FaC-^ V CHF 

| 1 


li diethylether ' 
/CF 




F 2 C\ 


1 1 , 
F 2 0\ ^CF 


F 2 




F 2 




F 2 
(major product) 


F 
(minor product) 










i 


[H-] 




F a 




F 




F 2 




F 2 CK X CH 

1 II 


+ 


1 1 * 


-F- 


- 1 T H 












J? aO\ >• CHF 




F s O\ /CHF 




F 2 




F 2 




F 2 




(major product) 




(minor product) 










U> [H-] 






(i) [H-] 








i 


(it) -F~ 




(H)-F- 








F 




H , 








F 2 C /( 


^CH 




X 
FsC^ 


^OH 











F 2 (V /CH 2 F 2 C X /CHF 

F 2 F a 

5. Cyelization Reactions. 96 Whereas ethylene and other simple alkenes do 
not dimerise to form cyclobutanes when heated, tetrafluoroethylene and 
perfluoropropene do. This remarkable property is shared by other ftuoro- 
olefins of the type CF 2 :CPR (R = H, CI, CN, CgHj, O.CH 3 etc.) and by 
a few of the type CF a :CR 2 - ^ n nearly every case studied, only the head- 
to-head dinner appears to be formed, e.g., 96 

2CF * :CFC1 7555^ I I „ < 80%) 

CF a — CFC1 

(approx. 50 : 50 cis and trans) 
200° CF 2 -CC1 2 

2OT ' S ^^5=E^ I I (92%) 

CFg CClg 



Both possible types of dimer are given by trifluoroethylene 97 and perfluoro- 

OF s -CF— CF 8 CF a -CF— CF» 



propene, 98 e.g., 



250-400 6 , 
CPs-CF:CF 2 -^Z* ! | + 



CF 2 -CF— CF 3 F,C— CF— CF 2 
{cis and trans) (cis and trans) 



46 



Perflnoro-Alkenes, -Alkadienes, -Cydoalkenes, and -Cycloalkadienes 

The dimerisation of tetrafluoroethylene to perfluorocyclobutane, which 
occurs smoothly at 200° (the reverse reaction takes place above 500°), is 
exothermic to the extent of ca. 50 kcal/mole, while the hypothetical cyclo- 
dimerization of ethylene should result in the liberation of only ca. 16 kcal/ 
mole. This difference can be attributed to an unusually large change in 
carbon bonding in passing from a C=C bond to a C — C bond in a fluoro- 
carbon system (as compared to a hydrocarbon system), and provides further 
evidence for the destabilisation of an olennic bond by fluorine substi- 
tuents." 

Tetrafluoroethylene co-dimerises with many non-fluorinated olefins, often 
more readily than it dimerizes to perfluorocyclobutane, e.g., 

F 2 C=CF2 CF 2 — CF 2 

+ . , > | I (40%) 

autoclave I i 

HaO =: CI£2 CH^ — CHg 

F S C=CF S CF 2 -CF a 

+ — ^-> I I (84%) 

autoclave I I 

H a C=CH.CN CH 2 -CH— CN 

and in reactions with conjugated dienes where a choice exists between the 
formation of a four- or a six-membered ring, the former is favoured, e.g., 

F 2 C=CF 2 CF 2 -CF 2 

+ . , > (90%) 

autoclave I I _ 

H 2 C=CH-CH=CH 2 CH 2 -CH— 0H=CH 2 

Acetylenes combine with tetrafluoroethylene to yield tetrafluorocyclo- 
butenes, which undergo ring-opening to give l,l,4,4-tetrafluorobuta-l,3- 
dienes when pyrolysed, e.g., 1 * 

F 2 C=CF 2 2250 CF 2 -CF 2 700 . /10mm 

+ — „ , > (35%) .„ . K > CF 2 :CH.CH:CFj (100%) 

autoclave I I silica tube 

HC=CH CH=OH 

F 2 C=CF 2 CF 2 -CF., 70 o«/lOmm 

+ 1 , > (73%) ,„ . . > CF 2 :C(C 6 H 6 ).CH:CF 2 

autoclave I I silica tube * , „,, 

C,H S — C^CH C«H 5 — C==CH (60%) 

Co-dimerizations between tetrafluoroethylene and other fluoro-alkenes can 
also be accomplished, e.g., 101 

F 2 C=CF 2 17so CF 2 — CF 2 

autoclave II 
F 2 C=CF— O-CHs CF 2 -CF— O-CH, 

These and analogous co-dimerization reactions involving other fluoro- 

47 



Aliphatic Flttorocarbons , 

olefins containing a CF 2 =C< group, together with the homo-dimerizations 
referred to above, provide the key to the synthesis of a vast array of fluo- 
rinated cyclo-bntanes and -butenes that would be extremely difficult to 
prepare by conventional methods; in addition, they provide intermediates 
for the synthesis of non-fluorinated substituted cyclobutene-3-ones and 
-3,4-diones, 102 and tropolone : i " 3 



F *o=cci 2 180 . op^oa, fl0i o=c— cci 2 

HCfeC— C«H, CH=C— C 8 H S CH=C-C e H 6 



F 2 C=CFC1 ,„„„ CFj-CFCl _ _„ 0=C C=0 

120° | , K-.H.BO. I I (75%) 

autoclave I I 100" II \ ' i 

HC^C— C«H 5 CH=C— C 6 H 6 CH=C— C,H 6 



F 2 C=CFC1 „„_„ CF 2 -CFC1 _ .. , CF a -CF 

200° 1 * 1 , „, . Zn, ethanol 111 

+ „ , > (80%) -r— > (100%) 

autoclave II heat I 11 

F 2 C=CFC1 CF 2 -CFC1 CF 2 -CF 



CH .QH, KOH aq. 1 2 Y v-vxit cone. H»SQ 4 

>• j j (72 /o) > 



CF 2 -C— O.0H 3 „ an HO— C— C=0 

I T , cone. HjSOi 11 1 

I I (72%) '—^ I I 

CF 2 -C— O.OH 3 HO— C— C=0 



U + L 



P F * 475°/Iatm. ^VGF 2 „ . CF 2 



+ 



Pyrex tube l^/.^ V/ CF 2 

I 



nic&el tube 



750°/5 mm 



F2 F 2 F2 F 2 

L 





120-130" 



CH 3 . COsK, CH 3 . CO2H, 
, , HsO (trace) 




(overall yield 20%) 



48 



Perfluoro-Alkenea, -Alkadienes, -Cycloalkenes, and -CycloaZkadienes 

It now seems generally accepted* 8,104 that the thermal homo- or co- 
dimerization of a polyfluoroalkene proceeds via a short-lived diradical 
intermediate in which the spins of the odd electrons are antiparallel and 
rotation about the Cl-2 and C3-4 bonds is possible before collapse to a 
cyclobutane occurs, e.g., 



CF S =CFCI 



CFj=CFC1 



CF^-CFCl 



CFj^-CFCl 

■ 4 

(A) 



. — n.r 



CF 8 -C< 



•CI 



CF 2 — C< 



CF g — C< 



J* 



+ 



•ci 

(co. 60:60) 



OF*— c/ 



CI 
CI 



CF S — CFCl 



FC1C— CF 2 



FC1C CF 2 

(B) 



FC1C— CFjj 
(C) 



The orientation of cyclo-addition can be predicted by choosing as inter- 
mediate the most stable of the possible diradicals; in the case shown above 
the expected order of stability is (A) > (B) > (C). On an experimental note, 
these thermal dimerizations are usually carried out under autogenous 
pressures in steel autoclaves in the presence of small amounts of free- 
radical scavengers such as hydroquinone^or terpene B, which inhibit poly- 
merization of the olefins but do not retard the cyclizations, presumably 
because these are not chain reactions. As pointed out earlier (p. 25), the 
manipulation of tetrafluoroethylene under pressure is hazardous owing to 
the exothermic nature of its homopolymerization and decomposition, 105 so 
great care must be taken to exclude oxygen and other polymerization 
initiators from vessels in which cyclo-addition reactions, or any others 
involving this olefin at super-atmospheric pressures, are carried out. Tetra- 
fluoroethylene stored under pressure should be inhibited with a terpene to 
preclude the possibility of a spontaneous explosion; the terpene can be 
removed by passing the gaseous olefin through a tube packed with silica 
gel. 

When perfmorobuta-l,3-diene is heated at 150-160° in a steel autoclave 
it is converted into a mixture of perfluorocyclobutene, dimers, trimers, and 
polymeric material 106 (cf. buta-l,3-diene, which, under similar circumstances, 
mainly dimerizes in Itiels-Alder fashion to give 4-vinylcyclohexene). The 
perfluorocyclobutene arises by cyclization of the fluoro-diene, 99,106 ' 107 an 
exothermic isomerization that can be effected quantitatively by flow 
pyrolysis of perfluorobutadiene at 500 °/l atm. The dimer fraction, the main 
product, is unsaturated and probably consists principally of the octa- 



49 



Aliphatic Flvarocarbons 

diene (XI) since it can be converted into perfluoro(tricyclo[3,3,0,0 2 ' 6 ]- 
ootane) (XII) in ca. 60% yield by thermal methods t™ 6 ' 108 



500°/l atm 



/ 

V 



flow pyrolysis 



I". 



i50-ifin° JPfi ^1? 

autoclave F 



o 

(XI) 



200° 



]? glass ampoule 




(XII) 



This novel type of dimerization is also believed to occur when perfluoro- 
(1,2-dimethyIenecyclobutane) (&ee p. 56) is heated: 109 



*• 



y 01 



Y* 







*v 


^2 


y 








150° a 








glass ampoule 









F. 



(82%) 



F 2 r a 



Perfluorocyclopentadiene readily dimerizes in Diels-Alder fashion to give 
perfluoro(tricyclo[5,2,l,0 2 '«]deca-3,8-diene) (XIII), 110 which, on the basis 
of 19 F n.m.r. measurements, 111 is believed to have the ewrfo-configuration; 
in this respect it resembles cyclopentadiene. Even at —22° perfluorocyclo- 
pentadiene undergoes 9% transformation into dimer during 24 hours, and 
at room temperature 62% conversion occurs during the same period, so 
it should be stored at liquid nitrogen temperature (—196°). In marked 
contrast to dioyclopentadiene, which readily regenerates cyclopentadiene 
when heated above 160°, perfluorocyclopentadiene dimer (XIII) is un- 
affected by storage at 475° in platinum for 45 minutes; when pyrolysed 
in a flow system at 680°/2 mm with a contact time of only 0-3 seconds, 



50 



Perfluoro-Alkenes, -Alkadienes, -Oyeloalkenes, and Cyoloalkadienes 

however, 76% decomposition occurs according to the scheme below. 110 So 
far no other perfluoro-l,3-diene has been reported to give a Diels- Alder 
dimer. 




F i -"i 



(XIII) 



Fr 



/ retro Diels- Alder 


fI 


W" 


v-" 


680°/2 mm / 




tf 




platinum \ 

tube \ 


*l 


3? 


F 


loss of 0F a bridge 


»l 


N^ky 


If 



:CFj 



(40%)+:CF al " '> CF 2 :CF a (38%> 



» V, 



(The above products are accompanied by unidentified material.) 

PerfluorocycLopentadiene partakes in the Diels-Alder reaction as a diene 
in its thermal reactions with ethylene, acetylene, butadiene, norbornadiene, 
maleic anhydride, dimethyl aeetylenedicarboxylate, and trifluoronitroso- 
methane, but as a dienophile when heated with anthracene, and as both 
a diene and a dienophile when treated with cyclopentadiene (see Fig. 2.8) ; 
even under forcing conditions it does not combine with tetrafluoroethylene, 
perfluoro- or perchloro-buta-l,3-diene, perchlorocyclopentadiene, or tetra- 
cyanoethylene, but is converted into its dimer (XIII), which is a byproduct 
in most of the successful Diels-Alder reactions shown in Fig. 2.8. 112 Per- 
fluorobuta-l,3-oiene, perfluorocyclohexa-l,3-diene, and perfluoro-(l,2-di- 
methyleneoycloputane) also combine with trifluoronitrosomethane to give 
the formal Diels-Alder adducts (XIV), 1W (XV), 11 * and (XVI), 115 respect- 
ively. However L only perfluorocyclohexa-l,3-diene 1M ' 117 has been reported 
to partake in Diels-Alder reactions with conventional dienes or dienophiles; 
perfluorobuta-li3-diene either does not react (e.g., as with maleic anhydride) 
or adducts believed to be cyclobutanes [e.g., butadiene -<• (XVII); acrylo- 
nitrile -+ (XVIII)] are formed. 118 Thermal reaction of perfluorobuta-1,3- 
diene with tetrafluoroethylene yields perfluoro (vinylcyclobutane) (XIX), 119 
so for the most part (known exceptions are self-dimerization and reaction 
with trifluoronitrosomethane) this diene undergoes thermal cyclization 

51 



Aliphatic Fluorocarbons 



■#■ 




Perfluoroalka-l,2-Diene8 

reactions typical of a polyfluoro-olefin of the type CF 8 :CFR (see p. 46). 
It has been suggested that steric interaction between fluorine substituents 
at CI and 04 prevents perfluorobuta-l,3-diene from adopting a planar 
cisoid conformation, so that it cannot readily partake in a Diels-Alder 
reaction as a diene. 112 




r op 



/"s 



F 

(XV) 




(XVI) 



I- CF:CF a Fj, 
-CH:CBL 



-CHiCH, 



•CF:CF t 
■CN 



-CF:CF, 



IF, 



(XVII) 



(xvni) 



(XIX) 



The generalisation, known as the 'Alder Rule', that a Diels-Alder re- 
action is facilitated when the diene contains electron-releasing groups and 
the dienophile electron-attracting substituents is given wide currency. Less 
well known is that the converse of the Rule is equally true : electron-poor 
dienes combine preferentially with electron-rich dienophiles. 120 The Diels- 
Alder reactions of perfraorocyclopentadiene are subject to this inverse 
electron-demand effect. Another interesting feature is that in the reaction 
between perfluprocyclopentadiene and cyclopentadiene, both dienes play 
the dual r61e of diene and dienophile (see Fig. 2.8), and a two-step mech- 
anism may be operative in which both adducts are formed by collapse of 
a common interjmediate. 121 



III. PEEFHJOBOALKA-l,2-DIENES 

t 

Detailed studies of the chemistry of perfluoroalka-l,2-dienes, i.e., per- 
fluoroallenes, which promise a rich harvest from the viewpoints of syn- 
thesis, mechanism, and utility of derivatives, have been initiated only 
recently. Progress has been hampered mainly by preparative difficulties and 
by the ease with which compounds of the type R3.R5.C : C : CF 2 (Rj. = per- 
fluoroalkyl; R^ = F or Rj.) dimerize, polymerize, or suffer attack by nucleo- 
philes. It should be recalled that the central carbon atom of the linear 
allenic system is sp-hybridized and linked to each of the adjoining trigonal 



53 



Aliphatic Fluorocarbons 

carbon atoms by a <r-bond and a zr-bond; the two n- bonds are not con- 
jugated with each other because they lie in planes that are mutually per- 
pendicular (see, for example, Fig. 2.9). 



■Y-2 V^-4-0^ 



o *„ 




Fig. 2.9. Tetrafluoroallene— bonding and geometry. 

A. Preparation 

Only five simple perfluoroallenes appear to have been synthesised: 
CF 2 :C:CF 2 , C 2 F 6 .CF:C:CF 2 , (CF 3 ) 2 C:C:CF 2 , CF 3 .CF:C:CF.CF 3 , and 
(CF 3 ) 2 C:C:C(CF 3 ) 2 . At present, the simplest of these, tetrafluoroallene 
(perfluoropropadiene), is best prepared as shown below, using commercial 
vinylidene fluoride and dibromodifluoromethane as starting materials; 
method B 122 is preferred to the original method (A 123 " 1Zi ) for effecting the 
conversion of l,3-dibromo-l,l,3,3-tetrafluoropropane into the allene, since, 
despite the extra stages, the overall yield is appreciably higher and ex- 
perimental difficulties associated with the use of potassium hydroxide 
pellets are avoided. Pyrolysis of disodium perfluoroglutarate does not yield 
tetrafluoroallene (see p. 59; cf. the preparation of perfluorobuta-l,3-diene 
from disodium perfluoro-adipate, p. 22). 



benzoyl peroxide 
CF 2 Br 2 +CH s :CF 2 -^ — *- CFjBr.CHjs.CFjBr. (51%) 



CF„Br.CH:CF 2 (89%) 

KOH pellets^-^~50°/30 cmHg KOH pellets -^IOO" (33J 

CFjjBr.CHj.CFjjBr Metliod A ^*-CF 2 :C:CF 2 

(705 
C-SiO, \*X>°H """Hg Jig tetrahydrofuran / V» 



_„ Br,, 34° 15.y— KOH aq. 

CF a Br.CH:CF J (86%) > CF s Br.CHBr.CF 2 Br (92%) > CF.Br.CBriCF. (91%) 

light 20' 1 6 cmHg 

Method B 
54 



Perfluoroalka-l,2-IHenes 

Perfluoropenta-l,2-diene, C 2 F 6 .CF:C:CF 2 , and perfluoro-(3-metbylbuta- 
1,2-diene), (CF 3 ) 2 C:C:CF 2 , are prepared by routes which stem from 
method A above and exemplify a potential general method of synthesis of 
allenes of the type R F RpC:C:CF 2 from commercial vinylidene fluoride, 
viz., 

BpR^XI + CKsiCFsj — 5^L_>. RpRi.CX.CH 2 .CF,I -^^ B r Bi.C:C:CF, 
or ligat 

(B* = perfluoroalkyl; BJ, = F or B p ; X = F or CI) 

Iodo-compounds of the type RpRpCXI are easily obtained by application 
of the Hunsdiecker reaction to silver perfluoroalkanecarboxylates (see 
p. 80), reaction of perfluoroalkenes with iodine monofluoride (see p. 80), 
and treatment of perfluoroalkenes or chlorotrifluoroethylene with per- 
fluoroalkyl iodides (see p. 81 and ref. 125). The actual reactions leading 
to the two allenes are given below. 

Perfluoropenta-lfZ-diene 13 * 

CF 3 I+CF 2 =CFCI u - v - U8h V CF 3 .CF 2 .CFC1I(69%) CH ' :C1?8 



145" 
CF 8 .CF 2 .CFC1.CH 2 .CF 2 I(89%) " ^ThT > C 2 F 5 .CF:C:CF 2 (68%) 

I'6rfiuoro-(Z-m,et}iyl}yuta-l,2-dienef z ' 

(CF S ) 2 CFI + CH 2 =CF 2 J~+ (CF S ) 2 CF.CH 2 .CF 2 I(86%) ^^hT* 

(CF 3 ) 2 C: C:CF 2 ( 18%)+ (CF S ) 2 CF.CH: CF 2 (33 % ) 
| eg. MX— KQH aq. | 
150°/10 cm Hg 

Perfluoropenta-2,3-diene, CF 3 .CF:C:CF.CF 3 , is produced in 8% yield 
when perfluoropenta-l,4-diene is left in contact with anhydrous caesium 
fluoride at 45° for eight hours (see p. 60), 128 and perfluoro-(2,4-dimethyl- 
penta-2,3-diene) can be obtained from perfluoro(dimethylketene) (see 

p . 97). 129, 130 

2<CF S ) 2 C:C:0 + (C 2 H 5 .0) S P -^ (CF 3 ) 2 C:C:C(CF 8 ) 2 (55%) + (C 2 H 5 .0) g P:0+CO 

(OF \ C f c\ 

olmlM-A "''"""""""v I I / „,< 600°/l-5 mm Hg 

2(CF 3 )2C:C:0 — — >■ (91%) ^> 

diglyme, 40° I I \ * ">/ silica tube 
(CF 8 ) 2 C=C— O 

(0F 8 ) 2 C:C:C(CF 3 ) 2 (95%) + C0 2 

Perfluoro(dimethylketene) can also be converted into the allenic acid 
fluoride (CF s ) 2 C:C:C(CF 3 ).COF. 129 

55 



Aliphatic Fhtorocarbons r 

B. Reactions 

Tetrafluoroallene is a colourless gas, b.p. —37-6° (cf. allene, b.p. —34-6°), 
which polymerizes readily at room temperature, especially if under pressure, 
and must therefore be stored at —196°. This polymerization, which is 
possibly initiated by radicals generated by disproportionation of the 
allene, 124 

2CF 2 :C:CF 2 ;=* CF 2 :CF.CF 2 . +CF; C.CF 2 - (R-, below, is one of these) 



R' ~r CF 2 sC*CF 2 



n-lCVFi 
R-CCF 2 - >■ 

II 
CF, 



— CF,.C— 



CF. 



can also be eliminated by storing tetrafluoroallene at room temperature in 
the presence of a free-radical scavenger (e.g., a terpene), but then slow 
dimerization occurs with the formation of perfluoro-(l,2-dimethylenecyclo- 
butane) (XX). 123,124 Presumably the latter reaction occurs by a diradical 
mechanism (cf. the formation of cyclobutanes from fluoro-olefins, p. 49), 
but it is difficult to assess the relative stabilities of the three possible inter- 
mediates (D — F). Because of the orthogonal arrangement of the jr-bonds 
in the monomer (Fig. 2.9), allylic p~7i overlap and stabilization are only 
attained in (D) and (F) if 90° rotation about their C— CF 2 - bonds occurs 
before ring-closure. The exclusive formation of the head-to-head dimer (XX) 
at the expense of the head-to-tail compound (XXI) does not, therefore, 
allow a decision to be reached regarding the identity of the intermediate 
except that it cannot be (F). Tetrafluoroallene co-dimerizes with 

•CF 2 — c=CF 2 



CF 2 -C=CF 2 
(D) 



2CF 2 :C:CF. 




CF 2 — C=CF 2 

CF 2 — C=CF 2 
(E) 

CF 2 — C=CF 2 




CF 2 — C=CF 2 



CFjf=C CF, 



CF 2 -C=CF 2 



(XXI) 



CF 2 — C=CF 2 
(F) 

perfluorobut-2-yne, m a rather rare type of reaction for an allene to undergo, 
and with trifluoronitrosomethane 182 (cf. the formation of an oxazetidine 



56 



Perfluoroalka - 1 , 2-Dienes 
from tetrafluoroethylene and this nitroso-compound, p. 141): 

/Knvl F »C-C=C-CF, CFa . ;0 F3C-N-O 

(50%) I I « — F 2 C:C=CF 2 * > | | (43%) 

F 2 C=C— CF 2 F 2 C=C— CF 2 

Perfluoro-(l,2-dimethylenecyclobutane) (XX) is a by-product in both these 
reactions. 

The other known perfluoroallenes containing a terminal :G:CF 2 group 
dimerize more readily than tetrafluoroallene and so must be stored as solids 
at low temperatures. In the absence of a free-radical polymerization in- 
hibitor, perfluoropenta-l,2-diene, b.p. 22-9°, dimerizes completely within 
two days at room temperature and no polymeric material is formed; the 
dimer consists of at least three isomers, two which have been assigned the 
provisional structures (XXII) and (XXIII). 126 Perfluoro-(3-methylbuta- 
1,2-diene), b.p. 19-7°, dimerizes more slowly than its non-branched isomer, 
and after storage as a liquid at room temperature for three-and-a-half 
days is only 15% converted into a complex mixture of dimers and oli- 
gomers. 127 

20 o C 2 F 6 .CF— C=CF 2 C 2 F 6 .CF— C-=0F 2 

8C t F..CF:C:CF t - =snsr | | (22%) + |. | (63%) 

CjFj.CF— C=CF 2 CF 2 -C=CF.C 2 F 6 

(XXII) (XXIII) 

The ease with which tetrafluoroallene polymerizes has forestalled attempts 
to determine the orientation of free-radical attack on the allenic system. 
However, photochemical hydrobromination of perfluoropenta-l,2-diene, 
which, unlike tetrafluoroallene (see below), is inert towards hydrogen 
bromide at 20° in the dark, yields mainly a mixture of 3iT-2-bromo-octa- 
fluoropent-1-ene (XXIV) and c*«-lfi r -2-bromo-octafluoropent-2-ene (XXV) 
(the trans-iaomer is also possibly present). This result indicates that bromine 
atom preferentially attacks the central carbon atom of the allenic system 
to give an intermediate radical (G) which exists long enough to permit 
rotation about the C2 — C3 bond and hence the formation of a resonance- 
stabilized allylic radical (H) : 134 

u. v. light 
HBr v H-i-Br- 

C 2 F 6 .CF:C:CF 2 +Br-^C 2 F 5 .CF^CBr:CF 2 ^[C 2 F e .CF.CBr:CF 2 -*-+ CaFB.CF^Br.CFj 



(H) 
HBr 



(G) 

CjjFj.CHF. CBr:CF 2 (42 %)+ C 2 F 5 .CF:CBr.CHF 2 (25 %) 
(XXIV) (XXV) 

57 



Aliphatic Fluorocarbons 

The most interesting property of tetrafluoroallene is probably its ability 
to react with either nucleophilic or electrophilic reagents under very mild 
conditions to yield 1 : 1 adducts. Thus it combines with neutral methanol 
at sub-zero temperatures to give the ether CH 3 .O.CF 2 .CH:CF a , 188 ' 136 and is 
rapidly converted into /?/?-difluoroacryloyl fluoride, presumably via the 
adduct (XXVI) (c/. p. 165), when treated with water : 131 

— HF 
CF^ttCFij+HgO > CF 2 :CH.CF 2 .OH ► CF 2 :CH.COF 

(XXVI) 

These results, like the quantitative formation of 2fl-pentafluoropropene 
from the allene and moist caesium fluoride at 100°, 124 



F" CF;rQ:=CF 2 ^CF s .C:CF 2 H '°> CF 3 .OH:CF 2 

reveal that nucleophilic attack occurs at a terminal carbon atom in tetra- 
fluoroallene. 

Nucleophilic destruction of tetrafluoroallene must drastically affect the 
yield of this compound in the last stage of its preparation by Method A 
(p. 54) ; fortunately the allene is a gas and can be swept from the reaction 
vessel in a stream of nitrogen as it forms. 

The susceptibility of tetrafluoroallene towards nucleophilic attack differs 
only in degree and not in kind from that expected for a perfluoro-olefin, 
so it is surprising to find that the allene yields the dichloride CF 2 C1.CC1:CF 2 
quantitatively when treated with an excess of chlorine at low temperatures 
in the dark and also combines smoothly and quantitatively with anhydrous 
hydrogen halides in the absence of light, catalysts, or solvents, and at room 
temperature or below, to give 2fl-pentahalogenopropenes, CF 2 X.CH:CF 2 
(X = F, CI, or Br). 124 The latter products display typical polyfluoro-olefin 
resistance towards electrophilic attack and do not react with the hydrogen 
halides under the conditions used. In view of the ease with which tetra- 
fluoroallene is attacked by nucleophilic species, it has been suggested 124 
that addition of a hydrogen halide involves attack by halide ion rather 
than protonation of the central allenic carbon atom. However, the pos- 
sibility that the reactions proceed by an electrophilic or a four-centre 
mechanism cannot be excluded and work is in progress with tetrafluoro- 
allene and other perfluoroallenes to help resolve this problem. 127 * 134> 136 

IV. PEBFLTXOKOAIiKYNES 

Compared with fluorocarbon olefins, relatively little is known about the 
chemistry of fluorocarbon acetylenes. Most of the published work concerns 

58 



Perfluoroalkynes 

perfluorobut-2-yne, CP3.C-C.CF3, the simplest readily-available perfluoro- 
alkyne. 

A. Preparation 

1. Difluoroacetylene. This acetylene has never been isolated and charac- 
terized although it has been suggested as a product of several pyrolytic 
or photolytic reactions. 137 ' 188 The most definite claim concerns the pyrolysis 
of difluoromaleic anhydride, 138 which is a good source of fluoropropiolyl 
fluoride : 

FC.CO 

III \ ~ «50°/I-2 mm Hg 

ll >° SSSTtab^-^ CFiCCOF+CF:0F+CO 2 + CO 

FCCO 

but no difluoroacetylene was isolated and its formation was deduced from 
mass spectrometry analyses. However, it is pertinent that pyrolysis of 
monofluoromaleic anhydride gives monofluoroacetylene in high yield. 139 
Pyrolysis of disodium tetrafluorosuccinate, which might have been ex- 
pected to undergo double decarboxylative defluorination and yield difluoro- 
acetylene, gives only tetrafluorosuccinic anhydride and trifluoroacryloyl 
fluoride; the latter product arises by thermal decomposition of the an- 
hydride, a reaction which is catalysed by fluoride ion and forms the final 
stage in a convenient laboratory synthesis of trifluoroacryloyl fluoride 
from commercial chlorotrifluoroethylene : 14 ° 

/ GF 2 .C0 2 H CF 2 .CO 

CF 2 :CF.COF(95%) + C0 2 (98%) 

2. Perfluoropropyne. Disodium hexafluoroglutarate, like the tetrafluoro- 
succinate, does not undergo double decarboxylative defluorination (to 
yield tetrafluoroallene) when pyrolysed, but gives mainly small amounts 
of CF 2 : CF.CF 2 .COF and, by fluoride-catalysed isomerisation of this product, 
CF 3 .CF:CF.COF. The latter acyl fluoride can be converted into perfluoro- 
propyne via pyrolysis of the derived sodium salt CF 3 .CF:CF.C0 2 Na, but 
a superior method of synthesis of this acetylene, the only one of its type 
(Rp.C.CF) known, starts from commercial vinylidene fluoride: 141 

CH * :0F * - (seeTsI) > OF 1 ,CBr.OT i B,(«%> ■ _^ 2Qa > CF a .CBr : CFBr(97%) 

(cia: trans = 60:40) 

Zn, dloxan 

-> CF s .O:CF(43%) 



heat 

6 ' 59 



Aliphatic Fluorocarbons 

Mercury-sensitized photolysis of perfluorocyclopropene also yields per- 
fluoropropyne. 159 The chemistry of perfhioropropyne (b.p. —50°) is cur- 
rently under investigation. 136 

3. Perfluorobut-2-yne. Perfluorobut-2-yne is usually prepared from per- 
chlorobutadiene as follows : 142 

CC1 2 :CC1.CC1:CC1 2 sbF '- sbF ' c H CF S .CC1:CC1.CF 3 <85%) - Zl ' C '"'' 0H > 
155° 3 3V ' heat 

CF 3 .C;C.CF a (co. 50%) 

It can also be prepared by fluorination of acetylenedicarboxylic acid with 
sulphur tetrafluoride, a toxic gas which is a specific reagent for the con- 
version of carbonyl and carboxyl groups into difluoromethylene and tri- 
fluoromethyl groups, respectively : 143 

»-^«*«n»^ CFt . C : . OTf(80%) 



170°, autoclave 



Of academic interest is the isomerization of perfluorobuta-l,3-diene to 
perfluorobut-2-yne in 83 % yield under the influence of anhydrous caesium 
fluoride at 100° in a sealed tube. 128 Several other perfluorodienes have been 
isomerized to acetylenes in similar fashion, indicating that perfluoro- 
acetylenes are more stable thermodynamically than their isomeric dienes, 
in contrast to the relative stabilities of the corresponding hydrocarbon 
systems. 128 ' 14 * For example, perfluoropenta-l,4-diene can be isomerized to 
perfluoropent-2-yne and under suitable conditions the dienic intermediates 
involved in the stepwise movement of unsaturation can be isolated : 128 

CsF 

CF 3 .0F a .C:C.CF 3 (48%) + 



/ 45 c 78 hr. 
/ CF 3 .CF:CF.CF:CF a (22%) + 

CF i! :CF.CF 2 .CF:CF 2 — / CF 3 .CF:C:CF.CF 3 (8%) 

\ CsF 



250°/40 sec. 



* CF S .CF 2 .C:C.CF 3 (85%) 



Evidence for transient perfluorocarbanion intermediates in these reactions 
is provided by the observation that retropropargyl rearrangement of per- 
fluoropenta-l,2-diene to perfluoropent-2-yne with caesium fluoride also 
produces a small amount of 2H-nonafluoropent-2-ene, presumably through 
the presence of traces of moisture in the system : 131 



'*-> C 2 F 5 .C:C.CF 3 



C 2 F 6 .CF:(P=CF 2 F — * C^-CF^-OFa— /jr 

x — »- C 2 F 5 .CF:CH.CF 3 

4. Bis(perfluoroalkyl)acetylenes in general. As a general method of syn- 
thesis of bis(perfluoroalkyl)acetylenes, fluoride-catalysed isomerization of 

60 



Perfiuoroalkynes 

dienes as outlined above is unsatisfactory owing to the difficulties associated 
with the preparation of starting materials. Thus normally one of the two 
general methods shown below is chosen; only the first can be used to 
prepare unsymmetrical alkynes, i.e., R F .C : .C.Rp, where R r and RJ. are 
different perfluoroalkyl groups. 



Perfluoralkyl iodide route 1 ** 



B P I+CH:CH -^> B p .CH:CHI i2!!i52= Rf .c;oh 



aw 



heat or u.v. light 



R*.CI:CH.Ri - aqae0ngalc0holleKOH > B F .C : C.R F 
Perfluoroalkanoyl chloride route 14 * 

2B F .COCl+2CF 2 :CCl, ****' 16 °°> 
autoclave 

R F .GF t .CCl:CCl.C¥ i .n T ^n, acetic anhydride > ^ ^ _ &; Q ^ ^ 

B. Properties and Reactions of Perfluorobut-2-yne 

Perfluorobut-2-yne is a colourless gas, b.p. —24-6° (c/. CH 3 .C : .C.CH 3 , 
b.p. 27-2°); as expected, it resists electrophilic attack and is susceptible 
to nucleophilic and free-radical attack (see Pig. 2.10). 1 * 7 When perfluoro- 
but-2-yne is heated under pressure, either alone or in the presence of small 
amounts of iodine, trifluoroiodomethane, or triphenylphosphine nickel 
carbonyl, it trimerizes to hexakis(trifluoromethyl)benzene in good 
yield, 148 - 180 e.g., 



3CF 3 c:c.CF s 



375"/25 atm. 



F.O- 



F a O' 




(69%) 



CF S 



CF S 



F.Cs 



FoC 



/ 



-OF, 
-CF„ 



CF 3 

(XXVII) 



^CF X 



X3F, 



CF. 



FaC> 



F S C | 



CF, 



F 3 c/ 



r 



F S C 



CF„ 



0F 3 



(XXVIII) 



One investigator claims that a tretramer believed to have either structure 
(XXVII) or (XXVIII) is also formed in this reaction. 160 Poly(perfluorobut- 
2-yne), which may have structure [— (CF 3 )C:C(CF 3 )— ]„, can be obtained 



61 



AUphatic Fhiorocarbons 



o 

u 

CQ 

o 
« 

O 



to, 

o 



S3 
O 









o 



o 

o 



a 
o 






s 




© 


C5 


2 




b 


>. 


n 


O 


c 


tu 


ffi 


CI 


U 


O 


S V 


£ 


M 


> Q 


o 




s 


CO 




o 


o 




« 


.<«■> 




th 


c» 




u 




!> 



t 



o 
© 

i-H 

c5* 

H 
ft 



62 



Perfltwroalkynes 

by subjection of the monomer to the action of high-energy radiation; 151 it 
is a white solid characterized by high chemical and thermal stability. 

Difluorocarbene, generated by gas-phase pyrolysis of the phosphorane 
(CF 3 ) 3 PF 2 (see p. 152) at 100°, adds across the triple bond in perfluorobut- 
2-yne to give perfluoro(l,2-dimethylcyclopropene), which in turn will 
combine with difluorocarbene to yield perfluoro(l,3-dimethylbieyelo[l,l,0]- 
butane): 162 

■ C3? * cw /^\ 
CF 3 .C:C.CF 8 — >■ CF S — C=C— CF S — — *-*■ CF 8 — C C— CF 8 

F 2 F 2 

Perfluorobut-2-yne is a powerful dienophile, as exemplified by its reac- 
tion with durene to form 2,3,5,6-tetramethyl-7,8-bistrifluoromethylbicyclo- 
[2 J 2,2]octa-2,5,7-triene : 188 



H 3 G- 
HsO 




CF 3 
CH 3 G 

+ c 

ch 3 9 

GF 3 



200" 



autoclave 



F 3 C 



CF. 



'/ 



I 



H 3 C<^ ^^CH 3 






(41%) 



Benzene itself does not combine with perfluorobut-2-yne at 200° under 
pressure, but at 250° reaction occurs with the production of a complex 
mixture containing trifluoromethyl-benzenes and -naphthalenes; the for- 
mation of these products can be explained 153 on the assumption that the 
Diels-Alder adduct (XXIX) is a transient intermediate in the reaction. 




(XXIX) 

Diels-Alder adducts have been obtained from perfluorobut-2-yne and 
butadiene, 2,3-dimethylbutadiene, 1S4 anthracene, naphthalene, and 2,3,6,7- 
tetrakistrifluoromethylnaphthalene. 153 

Several novel heterocyclic compounds have been prepared from per- 
fluorobut-2-yne. Thus when this acetylene is heated at 200° under pressure 
with red phosphorus and a catalytic amount of iodine, 2,3,5,6,7,8-hexa- 



63 



Aliphatic Fluorocwbons 

lristrifluoromethyl-l,4-diphosphabicyclo-octatriene (XXX) is obtained in 
43% yield; the analogous diarsine (XXXI) can be prepared in similar 
fashion and yield from arsenic and 2,3-di-iodohexafluorobut-2-ene, which 




(XXX) 




is made from perfluorobut-2-yne and iodine at 200°. 155 3,4-Bistrifluoro- 
methyl-l,2-ditbietene (XXXII) is formed in 80% yield when perfhiorobut- 
2-yne is passed through the vapours of boiling sulphur at atmospheric 
pressure, while reaction at 200° under pressure in the presence of iodine 
also produces tetrakistrifluoromethylthiophene (XXXIII) and a compound 
that has been formulated as a p-dithiino-p-dithiin (XXXIV). 186,167 



CF 3 .C:C.CF 8 + S 



200° 



s— s 



autoclave 



/ 



| | (26%) + 

CF 3 -C=C-CF 3 „ ^/^ a /\ 



CF 3 



(H%) 



(XXXII) 



F 3 C "S'' N CF S 
(XXXIII) 




(29%) 



The selenium analogue of the dithietene (XXXII) can be prepared from 
hot selenium vapour and perfluorobut-2-yne, 158 and tetrakistrifluoromethyl- 
p-diselenin (XXXV) is formed in low yield when 2,3-di-iodohexafluorobut- 
2-ene is heated with selenium at 180° under pressure. 158 



Se^/ 



CF, 



F 3 C X 



(XXXV) 



64 



References 
REFERENCES 

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65 



Aliphatic Fluorooarbona 

30. Errede, J. Org. Ohem., 1962, 27, 3425. 

31. Cady, Proc. Ohem. Soc, Lond., 1960, 133. 

32. Atkinson and McKeagan, Ghem. Gomm., 1966, 189. 

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39. Gozzo and Patrick, Tetrahedron, 1966, 22, 3329. 

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48. Henne and Newby, J. Amer. Ohem. Soc, 1948, 70, 130. 

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51. Banks, Haszeldine, and Walton, J. Ohem. Soc, 1963, 5681. 

52. Barbour, Mackenzie, Stacey, and Tatlow, J. Appl. Ohem., 1954, 4, 347; 
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53. Evans and Tatlow, J. Ghem. Soc, 1954, 3779. 

64. Park, Semx, and Lacher, J. Amer. Ghem. Soc, 1956, 78, 59. 
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56. Pattison, Toxic Aliphatic Fluorine Compounds, Elsevier, Amsterdam, 1959; 
various authors in Handbook of Experimental Pharmacology, Vol. XX/I (The 
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57. (a) Sohlag and Kaiser, J. Amer. Ghem. Soc, 1968, 87, 1171, and references 
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(b) Simons, Nature, 1965, 80S, 1308. 
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and Sharpe, Butterworths, London, 1965, Vol. 4, p. 50 and Dyatktn, Mooha- 
lina, and Knunyants, Russ. Ohem. Rev., 1966, 85, 417 for recent reviews of 
ionic reactions of fluoro-oleflns. 

59. England, Melby, Dietrich, and Lindsey, J. Amer. Ghem. Soc, 1960, 82, 5116. 

60. Wiley, U.S.P. 2,988,537/1961. 

61. Yarovenko and Raksha, J. Gen. Ghem. (U.S.S.R.), 1959, 29, 2125; Knox, 
Verlade, Bergbr, Cttadriello, and Cross, J. Org. Ohem., 1964, 29, 2187. 

62. Henne and Pelley, J. Amer. Ohem. Soc, 1952, 74, 1426. 

63. Dixon, J. Org. Ohem., 1956, 21, 400. 

64. Park and Fontanelli, J. Org. Ghem., 1963, 28, 258; Tarrant and Heybs, 
J. Org. Chem., 1965, 30, 1485. 

65. Rapp, J. Amer. Ohem.. Soc, 1951, 78, 5901; Prttett, Basner, and Smith, ibid., 
1952, 74, 1633, 1638, 1642; Ellzey and Gtjice, J. Org. Ohem., 1966, 81, 1300; 
Park and Frank, ibid., 1967, 32, 1336; Stookel and Megson, Ganad. J. 
Ghem., 1967, 45, 1998. 



References 

66. Koshab, Simmon's, and HrarMAjm, J. Amer. Chem. Soc, 1957, 79, 1741. 

67. Miller, Fbied, and Goldwhite, J. Amer. Chem. Soc, 1960, 82, 3091. 

68. Bbehm, Bremeb, Eleutebio, and Meschke, U.S.P. 2,918,501/1959. 

69. Graham and McCobmack, J. Org. Ghent., 1966, 31, 958. 

70. Gbaham and Weikmayb, J. Org. Chem., 1966, 81, 957. 

71. Fawcett, Ttjixock, and Coffman, J. Amer. Chem. Soc, 1962, 84, 4275. 

72. Smith, Fawcett, and Coffman, J. Amer. Chem. Soc., 1962, 84, 4285. 

73. Dbessleb and Young, J. Org. Chem., 1967, 32, 2004. 

74. Gibbs, U.S.P. 3,000,979/1961. 

75. Bbice, LaZebte, Hals, and Peablsok, J. Amer. Chem. Soc., 1953, 75, 2698. 

76. Haszeldine and Osbobne, J. Chem. Soc, 1956, 61. 

77. Bubdon and Tatlow, J. Appl. Chem., 1958, 8, 293. 

78. Milleb, Fbeedman, Fried, and Koch, J. Amer. Chem. Soc, 1961, 83, 4105. 

79. Miller and Fbeedman, J. Amer. Chem. Soc, 1963, 86, 180. 

80. Solomon, Dee, and Schults, J. Org. Chem., 1966, 31, 1551. 

81. Haszeldike, J. Chem. Soc, 1953, 3761. 

82. Stefant, Here, and Szwabc, J. Amer. Chem. Soc, 1961, 83, 4732; Stepani 
and Szwabc, Preprints of the Second International Fluorine Symposium, 
Estes Park, Colorado, U.S.A., July 1962, p. 304. 

83. Buech, Goldwhite, and Haszeldine, J. Chem. Soc, 1963, 1083. 

84. Habbis and Stacey, J. Amer. Chem. Soc, 1961, 83, 840. 

85. Stacey and Habbis, J. Org. Chem., 1962, 27, 4089. 

86. Case, Bay, and Roberts, J. Chem. Soc, 1961, 2070. 

87. For reviews see Bakes and Haszeldike, J. Oil Col. Chem. Ass., 1959, 42, 591 ; 
Bakes, Bibohall, and Haszeldike in Bigh Temperature Resistance and 
Thermal Degradation of Polymers, Society of Chemical Industry (London) 
Monograph, 1961, No. 13, p. 270; and Shebbatt, Kirk-Othmer Encyclopedia 
of Chemical Technology (2nd Edition), Interseienoe, New York, 1966, Vol. 9, 
p. 805. 

88. Eletttebio, tX.S.P. 2,958,685/1960; Eleutebio and Moore, Preprints of the 
Second International Fluorine Symposium, Estes Park, Colorado, U.S.A., 
July 1962, p. 344. 

89. Siakesi and Capobiccio, Makromol. Chem., 1963, 60, 213. 

90. Mttjjsb in Preparation, Properties and Technology of Fluorine and Organic 
Fluoro Compounds, ed. Slesser and Schram, McGraw-Hill, New York, 1951, 
p. 625. 

91. Bexfobd, U.S.P. 3,051,677/1962. 

92. Knunyants, Kbasuskaya, and Mysov, Izvest. Akad. Nauk S.S.S.R., Otdel. 
khim. Nauk, 1960, 1412; Kkukyants, KbasusJkaya, Mysov, and Muehtabov, 
ibid., 1962, 2141 ; Siakesi and Foktakelli, Ann. Chim. (Italy), 1965, 65, 850. 

93. Bakes, Bablow, Haszeldike, Lappik, Matthews, and Tucker, J. Chem. 
Soc. (C), 1968, 548. 

94. Feast, Pebby, and Stepheks, Tetrahedron, 1966, 22, 433 and references 
quoted therein; Bubton and Johkson, Tet. Lett., 1966, 2681. 

95. See Robebts and Shabts, Org. Reactions, 1962, 12, 1, and Huisgen, Gbashey, 
and Saueb, The Chemistry of Alkenes, ed. Patai, Interseienoe, New York, 1964, 
p. 779 for reviews of cycloaddition reactions of fluoro-alkenes. 

96. Solomon and Dee, J. Org. Chem-, 1964, 29, 2790; Hekke and Ruh, J. Amer. 
Chem. Soc, 1947, 69, 279. 

97. Fuller and Tatlow, J. Chem. Soc, 1961, 3198. 

98. Atkinson and Stockwell, J. Chem. Soc. (B), 1966, 740 and references quoted 
therein. 

99. Schlag and Peatman, J. Amer. Chem. Soc, 1964, 86, 1676. 

67 



Aliphatic Fluorocarbons 

100. Akdbesoit, Putnam, and Sharkey, J. Amer. Chem. Soc., 1061, 88, 382. 

101. Andbeades and England, J. Amer. Chem. Soc., 1961, 88, 4670. 

102. Roberts, Kline, and Simmons, J. Amer. Chem. Soc, 1953, 75, 4765; Smtjtny 
and Roberts, ibid., 1955, 77, 3420; Cohen, Laches,, and Pabk, ibid., 1959, 
81, 3480. 

103. Dbysdale, Gilbebt, Sinclair, and Sharkey, J. Amer. Chem. Soc, 1958, 
80, 3672. 

104. Wilson and Goldhameb, J. Chem. Educ, 1963, 40, 599; Babtlett, Mont- 
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Schttelleb, and Babtlett, ibid., p. 622; Baetlett and Montgomery, ibid., 
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1290; Hoffmann and Woodwabd, J. Amer. Chem. Soc, 1965, 87, 2046. 

105. Graham, J. Org. Chem., 1966, 81, 955; Shebbatt, Kirk-Othmer Encyclopedia 
of Chemical Technology, 2nd Edition, Interseience, New York, 1966, Vol. 9,p. 809. 

106. Prober and Muxes, J. Amer. Chem. Soc, 1949, 71, 598. 

107. Haszeldine and Osborne, J. Chem. Soc, 1955, 3880. 

108. Kable et al., J. Amer. Chem. Soc, 1964, 86, 2523. 

109. Banks, Deem, Haszeldine, and Taylor, unpublished results. 

110. Banks, Harrison, and Haszeldine, J. Chem. Soc. (C), 1966, 2102. 

111. Fields, Gbeen, and Jones, J. Chem. Soc. (B), 1967, 270. 

112. Banks, Harbison, Haszeldine, and Obbell, J. Chem. Soc. (O), 1967, 1608. 

113. Banks, Barlow, and Haszeldine, J. Chem-. Soc, 1965, 6149. 

114. Banks, Haszeldine, and Matthews, unpublished results. 

115. Banks, HIszeldine, and Taylor, J. Chem. Soc, 1965, 978. 

116. Chambers, Musgrave, and Pykb, Chem. & Ind., 1965, 564; Andebson, Feast, 
and MtrsGRAVE, J. Chem. Soc. (C), 1969, 211. 

117. Banks, Bridge, and Haszeldine, unpublished results. 

118. Ryazanova, Dolgopol'skii, and Klebanskii, Zhur. Vsesoyuz. Khvm. Obshch. 
im D.Z.Mendeleeva, 1961, 6, 356. 

119. Putnam, Anderson, and Sharkey, J. Amer. Chem. Soc, 1961, 88, 386. 

120. See Satter, Angew. Chem. (International Edition), 1967, 6, 16 for a recent 
comprehensive review of mechanistic aspects of the Diels- Alder reaction. 

121. Banks, Harbison, and Haszeldine, Chem. Comm., 1966, 338. 

122. Banks, Bablow, Davies, Haszeldine, and Taylor, J. Chem. Soc (C), 1969, 
1104. 

123. Jacobs and Bauer, J. Amer. Chem. Soc, 1959, 81, 606. 

124. Banks, Haszeldine, and Taylor, J. Chem. Soc, 1965, 978. 

125. Haszeldine and Steele, J. Chem. Soc, 1953, 1592. 

126. Banks, Bbaithwaitb, Haszeldine, and Taylor, J. Chem. Soc. (G), 1968, 2593. 

127. Banks, Bbaithwaitb, Haszeldine, andTAYLOB, J. Chem. Soc (C), 1969, 996. 

128. Milleb, Fbass, and Resnick, J. Amer. Chem. Soc, 1961, 88, 1767. 

129. England and Kbespan, J. Amer. Chem. Soc, 1966, 88, 5582. 

130. Chebttbkov, Abnov, and Kntjnyants, Bull. Acad. Sci. V.S.S.B., 1966, 559. 

131. Banks, Deem, Haszeldine, and Taylor, J. Chem. Soc. (G), 1966, 2051. 

132. Banks, Haszeldine, and Taylor, /. Chem. Soc, 1965, 5602. 

133. Banks, Bbaithwaitb, Haszeldine, and Taylor, unpublished results. 

134. Banks, Bbaithwaitb, Haszeldine, and Taylor, J. Chem. Soc. (C), 1969, 454. 

135. Banks, Haszeldine, and Taylor, Proc. Chem. Soc, Land., 1964, 121. 

136. Haszeldine et al., work in progress. 

137. Banks, Barlow, and Haszeldine, J. Chem. Soc, 1965, 6149; Banks, Haszel- 
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Tetrahedron Letters, 1967, 51l;idem,J.Org. Chem., 1967, 82, 3114;HEiCKLENand 
Knight, V.S. Govt. Bes. Bept., 1964, 39, 23 (cf. Chem. Abe., 1965, 62, 2699d). 



68 



I 



References 

138. MlBDMioir, U.S.P. 2,831,835/1958. 

139. Middleton and Shabkey, J. Amer. Chem. Soc, 1959, 81, 803. 

140. Banks, Bibchall, Clakke, Haszeldine, Stevenson and Isebson, J. Chem. 
Soc. (C), 1968, 266. 

141. Banks, Barlow, Davies, Haszeldine, Mullen, and Taylob, Tetrahedron 
Letters, 1968, 3909; J. Chem. Soc. (C), 1969(c), 1104; Banks, Bablow, and 
Mullen, J. Chem. Soc. (C), 1969, 1331. 

142. Henne and Finnegan, J. Amer. Chem. Soc, 1949, 71, 298. 

143. Smith, Angew. Chem. (International Edition), 1962, 1, 467 (review of SF 4 
chemistry). 

144. Dbesdneb, Tlttmac, and Young, J. Org. Chem., 1965, 30, 3524. 

145. Haszeldine, J. Chem. Soc, 1951, 588; ibid., 1952, 2504; Haszeldine and 
Leedham, ibid., 1954, 1634. 

146. Kbespan, Habdeb, and Dbysdalb, J. Amer. Chem. Soc, 1961, 83, 3424. 

147. Haszeldine, J. Chem. Soc, 1952, 3490. 

148. Bbown, Gewanteb, White, and Woods, J. Org. Chem., 1960, 25, 634. 

149. Habbis, Habdeb, and Sausen, J. Org. Chew., 1960, 25, 633. 

150. Habbis, U.S.P. 2,923,746/1960. 

151. Bbown and Gewanteb, J. Org. Chem., 1960, 25, 2071; Habbis, U.S.P. 
3,037,010/1962. 

152. Mahleb, J. Amer. Chem. Soc, 1962, 84, 4600. 

153. Kbespan, McKusick, and Cairns, J. Amer. Chem. Soc, 1961, 83, 3428. 

154. Putnam, Habder, and Castle, J. Amer. Chem. Soc, 1961, 88, 391. 

155. Kbespan, J. Amer. Chem. Soc, 1961, 83, 3432; U.S.P. 2,996,527/1961. 

156. Kbespan, J. Amer. Chem. Soc, 1961, 88, 3434. 

167. Kbespan and McKusick, J. Amer. Chem. Soc, 1961, 83, 3438. 

158. Davison and Shawl, Chem. Comm-., 1967, 670. 

159. Stuckey and Heicklen, J. Amer. Chem: Soc, 1968, 90, 3952 (see also Sab- 
gbant and Kbespan, J. Amer. Chem. Soc, 1969, 91, 415). 



69 



Chaptee 3 
DERIVATIVES OF PERFLUOROALKANES 



This chapter contains a fairly brief discussion of the chemistry of per- 
fluoroalkanecarboxylic acids and related fluorocarbon derivatives of the 
aliphatic series containing some common functional groups of organic 
chemistry. 1 The discussion is centred around perfluoroalkanecarboxylic 
acids because they are convenient starting materials for the synthesis of 
many fluorocarbon derivatives, including perfluoroalkyl iodides, which have 
proved invaluable in the development of the chemistry of perfluoroalkyl 
derivatives of the elements, the subject of the next chapter. 



I. PERFLUOROALKANECARBOXYLIC ACIDS AND 
DERIVED COMPOUNDS 

A. Preparation o! Perfluoroalkanecarboxylic Acids 

Two important general methods are available for the synthesis of per- 
fluoroalkanecarboxylic acids: electrochemical fluorination (see p. 11) of 
alkanoyl chlorides or fluorides and subsequent hydrolysis of the perfluoro- 
alkanoyl fluorides thus obtained, and oxidation of certain types of fluoro- 
olefin. 

Trifluoroacetic, pentafluoropropionic, heptafluoro-n-butyric, and per- 
fluoro-n-octanoic acids are prepared commercially in America by electro- 
chemical fluorination of the corresponding alkanoyl chlorides or fluorides : 



■r, „~-^ electrochemical _ HjO „ 

R.COX -— > Rf-COP — —+ Rp.CO.H 

fluorination 

(R, = CF S , C 2 F 6 , n-C 3 F 7 , n-C,F 16 ; X = CI or F) 

The yield of perfluoroalkanoyl fluoride decreases as the number of carbon 
atoms in the starting material increases : acetyl fluoride gives trifluoroacetyl 
fluoride in 85% yield, while n-butyryl fluoride gives a 36% yield of hepta- 
fluoro-n-butyryl fluoride together with 4 % of pentafluoropropionyl fluoride 
formed by degradation of the carbon chain with retention of the acid 
fluoride group. 

70 



Perfiuoroalkanecarboxylic Acids and Derived Compounds 

Trifluoroacetic acid is also prepared commercially by oxidation of 2,3-di- 
chlorohexafluorobut-2-ene obtained by fluorination of perchlorobutadiene 
with antimony chlorofluorides: 2 

cci^cci.cciicaa Sbg - sw ' cl ? 

2 * 165° 

CF 3 .CC1:CC1.CF 8 (crude, 95%) alka " ne ™ P °' ^ > 2 CF,.C0 2 H(87%) 

and perfluoroglutaric and perfluoroadipic acids can be prepared in analo- 
gous fashion: 3 



cic CCl „_ 

,, I, sut 


F 2 C CCl 

| || (50%) - 

r 2 C\ /cci 

F 2 


KMnOj aq. 
beat 


/CF 2 — CO a H 

► F 2 C (90%) 

N CF 2 — CQ 2 H 


II H 100-200°' 
C1C\ /CCl 
^C^ 

Cl 2 



a f 2 

CIC^ V CC1 -.„ FjC^ ^CCl _„ _ CF 2 — CF 2 — C0 2 H 

»i SbF 5 z | n , „, . KMnO« aq. i , „, . 
— *■ (44%) " > (83%) 
I 100-200° I II w/< " heat I s ' ' 

C1C X /CCl F 2 C\ /CCl CF 2 — CF 2 — C0 2 H 

c c 



CI F 



Permanganate oxidation of perfluorocyclobutene (see p. 35) affords a con- 
venient route to perfluorosuccinic acid. 

Difluoromalonic acid can be prepared from cMorotrifluoroethylene as 
follows p * 

NaCN, H.O/CH».CN H.0 

CF.:CFC1 J,U ' I1 ' U "' B '- > ' J > [CHFC1.CF,.CN] * - 



70-80°/2-7 atm "■ ' J in situ 



CHFC1.CF 2 .C0 2 H(> 63%)+ CHFCl.CF g .CO.NH 2 (>19%) 

t I 



OH- 

CHFC1.CF,.C0 8 H <* > 
u.v. light 

CFC1,.CF 2 .C0 2 H (86 %) ZZZZZ^b CF 2( C0 2 H) 2(51 %) 

or, more simply, as its diethyl ester by fluorination of diethyl malonate 
with commercially-available perchloryl fluoride : 5 

CH 2 (C0 2 .C g H 6 ) 2 C,H ; C ^ , 1 C 1°;:° H > OT,(0(V<W i (84%) 

Another route to trifluoroacetic acid is permanganate oxidation of the 
olefin CP 8 .CH:CHI, which is obtained in high yield when trifluoroiodo- 

71 



Derivatives of Perfluoroalkanes 

methane is heated or irradiated with acetylene, since there is little tendency 
for a propagation step to occur in the free-radical reaction that ensues: 6 

CF S I hea * > CF3.+I. 

3 or u.v. light 

CF,I CH = CH 
CF S . + CH;CH — > CF 3 .CH:CH. -+ CF 3 .CH:CHI+CF 3 . : ► etc. 

This synthesis of trifhioroacetic acid from trifluoroiodomethane is in itself 
of little value, since trifluoroiodomethane is usually made from the acid 
(see p. 79). Its importance lies in the fact that the general approach can 
be applied to longer-chain perfluoroalkyl iodides to give long-chain per- 
fluoroalkanecarboxylic acids : ' 

CF S I or C 2 F 6 I ^';°^> CF 3 .[CF 2 ] m .I (see p. 81) 



CF 8 .[CF 2 ] m .I+CH:CH heat ^ light > CF 3 .[CF 2 ] m .CH.-CHI 
CF S .[CF 2 L,.CH:CHI *"" ™" t °' '"> CF 3 .[CF 2 ] M .C0 2 H 

B. Properties and Reactions of Perfluoroalkanecarboxylic Acids 

Perfluoroalkanecarboxylic acids range in appearance from colourless 
liquids to white waxy solids ; they are extremely hygroscopic and, like fluoro- 
carbons, have unexpectedly high volatilities, high densities, and low re- 
fractive indices and surface tensions (c/. CF 3 .C0 2 H, b.p. 72-5°, d* 9 14890, 
ng 1-2850, y 20 15-Odyn/cm; CH 3 .C0 2 H, b.p. 118°, df 1-049, «f>° 1-3718, 
y» 27-6 dyn/cm). Due to the great electron-attracting power of a per- 
fluoroalkyl group (see below), perfluoroalkanecarboxylic acids are strong 
organic acids (cf. CF S .C0 2 H,8 #„ = i. 8; CC1 3 .C0 2 H, K a -= 0-9; CH 3 .C0 2 H, 
K a = 1-8 x 10- 6 ), although considerably weaker than strong inorganic 
acids (the Hammett acidity functions, H , for CF 3 .C0 2 H 9 and H 2 S0 4 
are —3-03 and -11-10, respectively), and form salts with ease. Most 
of the metal salts of the lower acids, including lead and silver salts, 
are readily soluble in water, acetone, and methanol; and the silver salts 
are soluble in ether and benzene. The alkali-metal salts of the higher 
acids are useful as emulsifying agents because of their ability to lower the 
surface tension of aqueous solutions enormously even when present in low 
concentration. 

Some reactions of heptafluoro-n-butyric acid, a typical perfluoroalkane- 
carboxylic acid, and of some of the compounds derived from it are shown 
in Fig. 3.1. Many of these reactions follow the normal course, sometimes 

72 



Perfluoroalkanecarboxylic Acids and Derived Compounds 




Derivatives of Perfluoroalkanea 

slightly modified by the inductive effect of the electronegative C 3 F 7 grpup, 
which also affects the properties of the products! For example: 

(a) The aldehyde n-C 3 F 7 .CHO, like chloral, forms a stable solid hydrate 
or aldehydrol, n-C 3 F 7 .CH(OH) 2 , which can be reconverted to the 
aldehyde by distillation with phosphorus pentoxide; the aldehydrol is 
cleaved by hot aqueous alkali to give 1 H- heptafluoropropane, 
CF 3 .CF 2 .CHF 2 , and formate ion. 10 

(b) The alcohol n-C 3 F 7 .CH 2 .OH (K a = 4-3 x lO" 12 ; cf. C 6 H 5 .OH, K a = 11 
x 10 -10 ) is about 10,000 times more acidic than n-butanol. 11 

(c) Ethyl heptafluoro-n-butyrate is easily hydrolysed : at 20° it is hydrolysed 
slowly by water and fairly rapidly by dilute alkali. 

(d) The amine n-C 3 F 7 .CH 2 .NH 2 (K h = 1-8 x 10-«; cf. n-C 4 H 9 .NH 2 , K h = 4-1 
x 10~ 4 ; C„H 5 .NH 2> K h = 4-2 x 10" 10 ) is a relatively weak base; dia- 
zotization of the amine hydrochloride at low temperature yields the 
diazoalkane n-C 3 F 7 .CHN 2 , which is sufficiently stable to allow cautious 
isolation. 12 

(e) Unlike nitroso-alkanes, heptafluoro-1-nitrosopropane does not form a 
colourless dimer, i.e. 

O^ N 3 F 7 

but exists only as a deep blue monomeric compound. 13 lake polyfluoro- 
nitroso-alkanes* in general, it participates in several interesting reactions 
(see p. 138) ; for example, it combines readily with tetrafluoroethylene 
to yield an oxazetidine (I) and a 1 : 1 copolymer (II) with chains com- 
posed of alternating monomer residues. 14 

n-C 3 Fj— N O 

.] . | [— N(n-C a F7)-O.CF 2 .CF 2 — ]„ 

CF2 t/Fg 

(i) (ii) 

Regarding the relative magnitude of the inductive effect of the perfluoro- 
n-propyl group, the results of calculations involving bond dissociation 
energies and i.r. absorption frequencies show that the effective electro- 
negativity of the trifluoromethyl group lies between that of chlorine and 
that of fluorine (electronegativities on Pauling's scale: CI, 3-0; CF 3 , 3-3; 
F, 4-0); therefore other perfiuoroalkyl groups would be expected to have 
similar high effective electronegativities. 15 

* A polyfluoro-compound is one that contains a high proportion of fluorine. The 
term polyfluoroalkyl denotes any alkyl group containing a high proportion of fluorine 
(e.g., CHF 2 , CF 3 .CHF.CF 2 , CF 3 .CFC1). 

74 



Perfluoroalkanecarboxylic 'Acids and Derived Compounds 

Several well-known reactions in hydrocarbon chemistry either do not 
take place or follow a new course when applied to fluorocarbon derivatives. 
For example: (a) It is impossible to prepare amides of perfluoroalkane- 
carboxylic acids by the action of heat on ammonium salts, e.g., 16 

^ t. •^ ™ 180-200° 
n-C 3 F 7 .C0 2 NH 4 v CF 3 .CF 2 .CHF 2 

(b) 1,1-Di-H-polyfluoroalkanols resist dichromate oxidation and react with 
phosphorus trihalides to yield trispolyfluoroalkyl phosphites, not poly- 
fluoroalkyl halides, e.g., 17 

n-C 3 F 7 .CH 2 .OH -^U (n-C 3 F 7 .CH 2 .0) 3 P not n-C 3 F 7 .CH 2 Cl 

(c) Perfluorocarboxylic acid amides, Rj,.CO.NH 2 , do not undergo the Hof- 
mann degradation with alkali hypohalite to give the corresponding primary 
amines, R F .NH 2 , which are still unknown (see p. 124). Thus heptafluoro- 
n-butyramide reacts with sodium hypochlorite or hypobromite to give 
high yields (85-95%) of the appropriate halide n-C 3 F 7 X (X = CI or Br), 
and is converted into sodium heptafluoro-n-butyrate by sodium hypoiodite. 
The Hofmann degradation of amides is believed to proceed as follows: 

NaOX OH- y- 

R.CO.NHa ■ > R.CO.NHX v R.CO.NX > 

t> nn'Ql ~ OH-,H»0 

<^°i *" RNCO : *■ R-NH a + CO» 

(R = alkyl or aryl) 

and the reason for the change in mechanism when R is a strongly electro- 
negative perfluoroalkyl group, R F , appears to be that the ion R F .CO.NX 
fails to rearrange with loss of X~ to give the isocyanate R F ,NCO. Thus 
when sodium i^-bromoheptafluoro-n-butyramide, [n-C 3 F,.CO.NBr]-Na+, 
prepared from the silver salt of the amide, is heated in aqueous solution,' 
it decomposes to yield 1-bromoheptafluoropropane, probably by a mech- 
anism which involves a 1,3-shift with elimination of cyanate ion: 18 

n-C 3 F 7 .CO.NH 2 ^^ n-C3F 7 .CO.NHAg(98o /o ) B "' CT ' co f 
n-C s F 7 .CO.NHBr(75o/ ) "»° H "»•> *-">; [n . C3 F 7 .CO.NBr]-Na + (99%) H '°' "^ 

— > n-C a F 7 Br(91%) + NGO- 



n-C 3 F 7 — 0=0 

Br— N- 



75 



Derivatives of Perfluoroalkanes 

However, when the anhydrous sodium salt is heated, heptafluoro-n-propyl 
isocyanate is obtained in high yield, i.e. the key step in the normal Hof- 
mann degradation appears to occur: 



11-C3F7— c=o 

I < 170710- 2 mm ,, r ,. 

_ -N— j— Br _». NaBr-f n-C 3 Fj— C— N: > n-C 3 F 7 .NCO{83%) 

• Na+ 




and it has been pointed out that under these circumstances the sodium 
ion is held in close proximity to the i^-bromoheptafluoro-n-butyramide 
ion in the crystal lattice of the anhydrous salt and is thus able to play an 
essential role in the reaction. 1 * 

By contrast with the Hofmann degradation of perfluoro-amides, the 
Curtius rearrangement of perfluoroalkanoyl azides, R F .CO.N 3 , proceeds 
normally to give perfluoroalkyl isocyanates in high yield (e.g., see Fig. 3.1). 
This is not unexpected, since the Curtius reaction involves loss of a neutral 
molecule (nitrogen) in the key step (R.CO.N 3 -<- R.CO.N: + N 2 ; as in the 
analogous step in the Hofmann degradation, the migration of group R may 
synchronize with the departure of the leaving group), rather than loss of 
an anion, and such a process will clearly be considerably less sensitive to 
the electron-attracting power of the group R. 19 

The reactions of perfluorinated dicarboxylic acids follow from those of 
their hydrocarbon counterparts and monofunctional analogues. 20 The 
main work on the application of the Kolbe reaction to perfluoro-acids has 
been carried out with dicarboxylic acids, which electrolyse in normal 
fashion, e.g., 21 

H0 2 C.[CF 2 ] 3 .C0 2 CH 3 eleCtr ;f; i ^ C d ° 3 -° H > CH 3 O a C.[CF a VC0 2 CH 3 (57%) 
(partly as Na salt) 

CF 3 .C0 2 H+HO a C.[CF a ] 3 .C0 3 CH 3 ""^J^™ CF 3 .[CF 2 ] 3 .C0 2 CH 3 (36%) 

(partly as Na salts) 

C. Uses of Trifluoroacetic Acid, Trifluoroacetic Anhydride, and 
Peroxytrifluoroacetic Acid in Organic Chemistry 

Trifluoroacetic acid is the most important and readily available perfluoro- 
carboxylic acid. In addition to being the starting point in the synthesis of 
many trifluoromethyl derivatives, mainly through its ready conversion to 
trifluoroiodomethane (see p. 79), this acid and, in particular, its anhydride 
are important reagents in organic chemistry. 22 

76 



Perfluoroalkanecarboxylic Acids and Derived Compounds 

Trifluoroacetic acid is a colourless, hygroscopic liquid, b.p. 72-5°, with 
a sharp biting odour; it forms a maximum-boiling azeotrope with water 
(20-6% H 2 0; b.p. 105-5°) and is a good solvent for a wide variety of ali- 
phatic and aromatic compounds. It is stable at temperatures up to 250°, 
and studies in the range 300-390720-200 mm Hg indicate that thermal 
breakdown occurs largely via elimination of hydrogen fluoride followed 
by the formation of carbon dioxide and difluorocarbene, which mainly 
reacts with unchanged trifluoroacetic acid to yield difluoromethyl trifluoro- 
acetate. 28 Pyrolysis of sodium trifluoroacetate in the presence of sodium 
hydroxide at 270° also appears to give difluorocarbene, since tetrafluoro- 
ethylene is produced in at least 32% yield; 16 pyrolysis of sodium trifluoro- 
acetate alone yields trifluoroacetyl fluoride and trifluoroacetic anhydride. 24 
In contrast to sodium trifluoroacetate, sodium chlorodifluoroacetate is an 
important liquid-phase source of difluorocarbene, since it decomposes 
smoothly when heated at relatively low temperatures in an aprotic solvent 
and many examples of its use in synthesis have been reported. 25 

Trifluoroacetic anhydride, b.p. 40°, obtained by dehydration of the acid 
with phosphorus pentoxide, is used extensively to promote esterifications, 
since when it is added to a weak oxy-acid a mixed anhydride is formed, 
the reactions of which can be simply rationalized on the basis that in the 
presence of the trifluoroacetic acid formed concomitantly it ionizes slightly 
to provide an acylium ion, which is a powerful acylating agent, e.g., 

(CF 3 .CO) 2 O+0H 3t CO a H —y 

CF 3 .C0 2 H+CF 3 .CO.O.CO.CH 3 ^t CF 3 .COj+CH 3 .CO+ 

For the simplest acids and alcohols the esterification reaction is spon- 
taneous and rapid, but with less reactive acids and hydroxy-compounds 
gentle warming is required to complete the reaction. Primary, secondary, 
and tertiary alcohols, polyfluoro-alcohols, polyhydroxy-alcohols, nitro- 
alcohols, phenols, and thiophenbls have been esterified by this technique, 
and the acids used include carboxylic acids, sulphonic acids, phosphoric 
acids, and nitric acid. Since trifluoroacetic anhydride-promoted esterification 
X)f hydroxy-compounds requires only mild reaction conditions, it is a 
particularly valuable method for the preparation of esters of acid-sensitive 
compounds, and thus finds use in synthetic carbohydrate chemistry. 

Esters of trifluoroacetic acid are best prepared by heating gently under 
reflux a mixture of the alcohol, trifluoroacetic anhydride, and sodium tri- 
fluoroacetate, but sometimes the reaction occurs spontaneously at room 
temperature. Through the electronegativity of the trifluoromethyl group, 
trifluoroacetate esters readily revert to the parent alcohols on treatment 
with water or methanol at room temperature; this fact, coupled with the 
ease of preparation of trifluoroacetates under mild conditions, has led to 
the use of the trifluoroacetyl group as a protecting group in carbohydrate 

77 



Derivatives of Perfluoroalkanes 

and nucleic acid chemistry. For similar reasons, the trifluoroacetyl group 
is employed as a jV-blocking group in peptide chemistry and in work with 
amino-sugars. 26 Trifluoroacetic anhydride will, also acylate reactive aromatic 
compounds (e.g., 27 pyrrole, thiophen) without the aid of a Friedel-Crafts 
catalyst. 

Peroxytrifluoroacetic acid, CF 3 .CO.O.OH, is a very powerful oxidizing 
and hydroxylating agent; under mild conditions and often in excellent 
yields, it will convert i^-nitrosoamines to i^-nitroamines (R 2 N.NO -* 
RaN.NOis), 28 oximes to nitroparaffins (R 2 C:NOH -* R 2 CH.N0 2 ), 29 anilines 
to nitrobenzenes (Ar.NH 2 -> Ar.NOg), 30 olefins— including some with nega- 
tive substituents — to epoxides which, depending on the conditions, are 
either isolated as such 31 or as hydroxytrifluoroacetates that readily undergo 
acid-catalysed methanolysis to yield 1,2-glycols 32 



/°\ 




OH 

I I 

>c=c< — >■ >c — c< — ► — c— c 

I I 

CF s .C0.0 

ketones to esters (R.CO.R' -»- R.C0 2 R'), 3S and benzene homologues to 
phenols 34 (CF 3 .C0 3 H — BF 3 is a particularly effective reagent for effecting 
electrophilic aromatic hydroxylations 35 and can also be used to convert 
substituted olefins into ketones in one step, 36 e.g., tetramethylethylene into 
methyl t-butyl ketone). It shows many advantages over hydrocarbon 
peracids or other reagents that can be used to effect these conversions, but 
is a relatively expensive chemical. The best method for the preparation of 
peroxytrifluoroacetic acid is slow addition of trifluoroacetic anhydride to 
a stirred suspension of 90 % hydrogen peroxide in methylene chloride cooled 
to 0°; the solution of peracid thus obtained is normally used directly: 

(CF 3 .C0 2 )0+H 2 2 ethylene chloride aPt.GOJl+CBt.OOJl 



Trifluoroacetic acid can be employed with advantage to catalyse a 
number of condensation reactions, to promote olefin alkylation and hydra- 
tion reactions, and as a catalyst for Beckmann rearrangements. In the 
presence of its anhydride (to remove the water formed) it reacts with lead 
oxide to give lead tetrakis(trifluoroacetate), a moisture-sensitive, white, 
crystalline solid which may prove to be of interest as a reagent in organic 
chemistry; so far, it has been shown to convert benzene and heptane into 
trifluoroacetates under conditions where they will not react with lead tetra- 
acetate. 37 



78 



Perfluoroalkyl Iodides 

II. PERFLUOROALKYL fODIDBS 

Perfluoroalkyl iodides are highly reactive compounds and synthetic 
intermediates of great importance in fluorocarbon chemistry. By contrast, 
perfluoroalkyl bromides and chlorides are relatively unreactive compounds, 
especially the latter since they approach the saturated fluorocarbons in 
inertness, and only a few attempts to use these halides as intermediates 
in synthesis have been reported. 

A. Preparation 

The first fluorocarbon iodides, trifluoroiodomethane and pentafluoro- 
iodoethane, were prepared, as shown below, in the late 1940's at Cambridge 
University by Emeleus, Haszeldine, and their co-workers, who used them 
to obtain trifluoromethylmercuric iodide and its pentafluoroethyl analogue 
and thus initiated their extensive researches on perfluoroalkyl derivatives 
of various elements : M 



CI 4 m " 2ft - 10 °°> CF 3 I(9 5 %) 

Tpi ft -I ono 

CI 2 :CI 2 - >- CF a .CF 2 I(30%) 

I 2 + CF 2 :OF 2 ^ > CF 2 I.CF 2 I(76%) "" 90 ~ 100 '' > . CF..CF 2 I(85%) 

autoclave * 3 e 

R F .I + Hg heat °* ■ > R F .HgI(80-88%) 

u.v. light 

(R p = CF, or C 2 F S ) 

The original method of preparation of trifluoroiodomethane was later 
abandoned in favour of a much more convenient method that is still used 
today, namely the careful pyrolysis of an anhydrous mixture of silver 
trifluoroacetate and an excess of iodine : 

100° 
CF 8 .C0 2 Ag+I 2 v CF 3 I(>90%)+GO a + AgI 

This method was introduced independently by Henne and Finnegan 39 and 
by Haszeldine 40 in 1950; the type of reaction involved is a general one for 
silver salts of carboxylic acids, the so-called Hunsdiecker reaction: 

heat 
R.C0 2 Ag+X 2 >- R.X+C0 2 +AgX 

(R = alkyl or aryl, X = CI, Br, or I) 

79 



Derivatives of Perfluoroalkanes 

and can be applied to any silver perfluoro-carboxylate and involve the 
use of chlorine, bromine, or iodine, 41 e.g., 

ci,.ioo°^ n . Ci . FiiC1(7I o /o) 



n-C 5 F u . C0 2 A g - < Br " 80 " 9 °° > n-C 5 F u Br(83 % ) 

\ I " 100 ° > n.<vr u i(74%) 

Ag0 2 C.[CF 2 ;i 1 .CO i! Ag u - 100 ° > i.[CF 2 ] 4 .I(64%) 

F 2 F 2 

F 2 C^ N3F-C0 2 Ag T .„„ F 2 x X CFI 

| | -^U I I (63%) 

F 2 C\ /CF 2 F 2 C-v /CF 2 

F 2 F 2 

These reactions proceed via formation of acyl hypohalites, e.g., 

CF 3 .C0 2 Ag+I 2 —y CF 3 .CO.OI+AgI 

CF3.CO.OI — ► I-+CF 3 .CO.O- — ► C0 2 +CF 3 - — — '-+ CF 3 I+CF 3 .CO.O- — > etc. 

and if carried out under suitable conditions they can be arrested at this 
stage and the hypohalite used to procure nuclear halogenation of benzene 
and its derivatives, since it is a powerful source of 'positive' halogen due 
to the inductive effect of the perfluoroalkyl group. 42 

Since the perfluoroalkanecarboxylic acids CF 3 .C0 2 H, C 2 F s .C0 2 H, 
n-C 3 F 7 .C0 2 H, and n-C 7 F 15 .C0 2 H are commercial products, the most con- 
venient fluorocarbon iodides to prepare are the perfluoro-1-iodoalkanes 
CF3I, C 2 F 5 I, n-C 3 F 7 I, and n-C,F 15 I; and of these iodides only CF 3 I and 
n-C 3 F 7 I are commonly used in academic studies, doubtless because the 
acids from which they are prepared by the Hunsdiecker reaction are the 
cheapest.* An alternative route to pentafluoroiodoethane lies in the two- 
stage process referred to earlier (see p. 79) or, preferably, in a related 
process reported more recently :** 

'IF' from Iir. + 21^ ^ ^^ , 



autoclave 

Interestingly, pentafluoroiodoethane is formed in 7% yield when tetra- 
fluoroethylene is heated with potassium fluoride and iodine in acetonitrile 
solution at 150° under pressure; while this type of reaction, which may 

* Approximate U.K. prices for perfluoroalkanecarboxylic acids are : CF 8 .C0 2 H, 
£5 per lb; C 2 F 5 .C0 2 H, £17 per lb; n-C 3 F,.C0 2 H, £9 per lb; n-C,F 15 .C0 2 H, £41 
per lb. 

80 



Perfluoroalkyl Iodides 

proceed via transient perfluorocarbanions, is of little value for the prepara- 
tion of pentafluoroiodoethane, it is useful for the preparation of secondary 
fluorocarbon iodides, e.g., 43 

CF 3 .CF:CF 2 ".^jy**,, CT ,.CFI.OT,(61%) 

CF 3 .CF:CF.CF, **■ ^tonltrile ; C F 3 .CF 2 .CFI.CF 3 (17%) 

Heptafluoro-2-iodopropane, CF 3 .CFI.CF 3 , can be prepared in much higher 
yield (99%) by the interaction of perfluoropropene with a mixture of iodine 
and iodine pentafluoride (as a source of 'IF') at 150°. 44 

Reaction of perfluoroisobutene with iodine and iodine pentafluoride in 
the presence of aluminium catalysts at 130° gives perfluoro-t-butyl iodide 
in 69% yield. 45 

A complete series of perfluoro- 1 -iodoalkanes of general formula CF 3 . [CF 2 ]„.I 
can be prepared by the telomerization of tetrafluoroethylene with trifluoro- 
iodomethane (to give the odd members of the series, CF 3 .[CF 2 .CF 2 ] B .I) and 
with pentafluoroiodoethane (to give the even members of the series, 
CF 3 .CF 2 .[CF 2 .CF 2 ]„.I): 4 « 

„ T u.v. light 

Rp.I >• Bf* + I* 

Initiation 

Rp* -j-CF2:CF 2 — ► Rp.CF2.CFg* 

Propagation 

R F .CF 2 .CF 2 . + (n-l)CF 2 iCF 2 — v Rp.[CF 2 .CF 2 ]„. 

Chain transfer 

R P .CF 2 .CF 2 . + R P .I — > Rp.CFj.CFj.I+Rp,. - CT ' :Cg % etc . 

Rp.[CF 2 .CF 2 ]„. +R F .I — > R F .[CF 2 .CF 2 ]„.I+R,. CF ' :CF » > , etc. 
(R p = CF 3 or C 2 F 5 ) 

The value of n can be controlled by varying the proportion of R r I in the 
reaction mixture. Thus a molar ratio of trifluoroiodomethane to tetra- 
fluoroethylene of 10:1 results in a 94% yield of heptafluoro-1-iodopropane 
{n = 1), whereas an equimolar mixture gives products with n = 1, 2, 3, 
and > 3 in 16, 10, 5, and 63% yield respectively. Since a perfluoroalkyl 
iodide CF 3 .[CF 2 ] m .I can be converted into the next but one higher homo- 
logue CF 3 .[CF 2 ] ra+2 .I in high yield by reaction with a small amount of 
tetrafluoroethylene, the optimum method for the synthesis of the long- 
chain iodides is to proceed stepwise, i.e., C 3 F V I — C 5 F n I, C 5 F n I — C 7 F 16 I, 
etc. 

81 



Derivatives of Perflworoalkanes 

B. Reactions 

1. Ionic Reactions. Chemically, trifluoroiodomethane (b.p. — 22-5°) is quite 
different from methyl iodide (b.p. 42-5°), since the three fluorine atoms 
shield the carbon atom from nucleopbilic attack and through their in- 
ductive effect appear to cause the C — I bond to be polarized in the opposite 
direction to that found in methyl iodide. 

R ,_,_ H \s+ «- 

Thus trifluoroiodomethane undergoes S N 2 reactions of the type 

Nu- CF 3 — i y CFsNu+I- 

with only the greatest difficulty, if at all : it does not react with aqueous 
potassium hydroxide, moist silver oxide, potassium phthalimide, or metallic 
cyanides or nitrites at temperatures below 150°, and at higher temperatures 
complete destruction of the molecule occurs with liberation of fluorine as 
fluoride ion. 47 However, treatment of trifluoroiodomethane with acetonic 
or alcoholic potassium hydroxide at room temperature converts it into 
fluoroform in high yield, and this reaction appears to involve nucleophilic 
displacement on the 'positive' iodine to form hypohalite and the trifluoro- 
methyl anion, which rapidly abstracts a proton from the reaction medium : 4 ' 

/~>l P* solvent 
OH- 1— GF 3 > HOI + CF3- >- CHF 3 

As will be seen later (Chapter 4), perfluoroiodoalkanes will undergo 
reactions which involve heterolytic fission of the C — I bond, notably the 
formation of organometallic compounds, in the presence of solvents such 
as ethers and amines, i.e. neutral-molecule Lewis bases. From the results 
of u.v. spectroscopic studies, it appears that such bases, B, combine with 
perfluoroiodoalkanes, R F I, to form molecular complexes of the type (R F I).B 
in which there is transfer of an electron from B to Rjl with formation of a 
weak covalent bond between the odd electron of B+ and (Rj.I)~ . It seems 
that in one of these complexes the C — I bond approaches more to that in 
an iodoalkane in character, so that it will undergo much more readily 
reactions which involve ionic intermediates. 48 

Reasoning similar to that used above for trifluoroiodomethane can be 
employed to explain why other perfluoroiodoalkanes resist nucleophilic 
substitution on carbon attached to iodine, which is removed as positive 

82 



Perfluoroalkyl Iodides 

halogen by ethanolio potash to yield monohydrofluoroalkanes, RjH. 46 Such 
monohydro-compounds ('fluorocarbon hydrides') can also be prepared by 
catalytic hydrogenation of perfluoroiodoalkanes :** 

^ T Ha/Kaney HI „ „ 

BpI 3500/60 atn,.' ^H(>80%, 

by thermal decarboxylation of alkali-metal perfluoroalkanecarboxylates in 
suitable protic solvents : 49 



Bp.COJ * > RJ H+ d °°° r > R F H(60-98%) 
-C0 a e.g. glycol 



or by indirect hydrofluorination of perfluoro-olefins (see p. 33) : 



R P R P C:CF 3 F > R P R F C.CF 3 H+ d0nOr > R P R P CH.CF 3 (~60%) 
(R F = perfluoroalkyl; R P = F or Rp) 



Fluorocarbon hydrides are carbon acids with strengths which vary according 
to the nature of the fluorocarbon group. For example, 60 the following order 
of acid strengths has been established by measurement of rates of sodium 
methoxide-catalysed hydrogen-deuterium exchange in methanol-O-rf: 
(CF 3 ) 3 CH > (CF 3 ) 2 CHF > CF 3 .[CF 2 ] 6 .CHF 2 > CHF 3 [relative rates: 
1 x 10 9 , 2 x 10 5 , 6, 1-0; estimated pK a values on the Streitwieser acidity 
scale: 11, 18, 27, 28, respectively (c/. 81 cyclopentadiene, 15; fluorene, 22-9; 
triphenylmethane, 31-5)]. The dramatic changes in exchange rates in 
passage from primary to tertiary fluorocarbon hydrides has been discuss- 
ed 80 ' 61 in terms of the relative stabilities of the intermediate perfluoro- 
carbanions, with tertiary > secondary > primary > CF, . It follows from 
this order that /5-fluorine is far more carbanion-stabilizing than ar-fluorine, 
a situation initially attributed to enhancement of the inductive stabilization 
from the /S-position by a negative hyper conjugation effect, 50 ' 81 e.g., 

F \I/ F F \]/ F F \|/ F F \i/ P 

LI II 

F \ / c \ / F *"* F \ / c ^ F " / c \ / F ~ / c ^ F " 

p/ | I \p f./ | | Np I x F | \f 

F F F F F F 

9 equivalent 6 equivalent 

structures structures 

83 



Derivatives of Perfluoroalhanes 






*2 




*2 




K 




A/ F 




V^Cv- 


Y 


=o 


7 F * 




V 2 jC- 


-^^ 





*^F 2 Z^^ 8 



mr (IVJ 



However, the six-/J-fluorine tertiary hydride lH-undecafluorobicycIo[2,2,l]- 
heptane (III) undergoes base-catalysed proton exchange five times faster 
than (CP 3 ) 3 CH despite the fact that its conjugate base (IV) is forced to 
remain pyramidal with consequent inhibition of carbon-fluorine no-bond 
resonance in accordance with Bredt's rule. 53 This, taken in conjunction 
with the results of an investigation of the kinetic acidity of 9-trifluoro- 
methylfluorene, 84 has been quoted as compelling evidence against the 
occurrence of fluorine hyper conjugation as a significant stabilizing pheno- 
menon in fluoroalkyl anions. Accepting this, the stabilities of perfluoroalkyl 
carbanions must derive largely from inductive effects; relative stabilities 
can then be rationalized by recalling that an a -fluorine does not stabilize 
a carbanion (> CF) to the extent that would be expected from consideration 
of its powerful ff-electron-attracting properties ( — I effect),** presumably 
owing to the low polarizability of fluorine and repulsion between the 
fluorine lone-pair electrons and the electron pair on the negative carbon. 
The magnitude of the repulsive effect would depend on the geometry of 
the carbanion in question, optimum repulsion being realized if the negative 
carbon were sp 2 -hybridized with the electron pair in a 2p-orbital — an 
improbable situation since the tendency for the charge to occupy orbitals 
as rich in s- character as possible tends to make carbanions pyramidal, with 
s^-hybridized negative carbon. 51 It has been noted 53 that compound (III) 
may possess enhanced acidity because of increased s-character in the 
C — H bond derived from ring-strain effects. 

The ease of removal of proton from tris(trifluoromethyl)methane (slow 
H — D exchange occurs in boiling eth&nol-O-d), coupled with the relative 
ease of synthesis of this hydride, 56 enables it to be used to introduce the 
(CF 3 ) 3 C group into structures susceptible to earbanionic attack, e.g., 67 

* This compound is one of the products produced by fluorination of bicyclo- 
[2,2,l]hepta-2,5-diene with cobalt trifluoride at 250-300°. 5!! 

** For example, the halogen substituents of haloforms facilitate carbanion forma- 
tion in the order I ~ Br > CI > F, i.e. in almost the reverse order expected from 
the electronegativity order F > CI > Br > I. This has been explained in terms of 
the combined effect of inductive withdrawal of electron density, halogen polariza- 
bility, and the possibility of d-orbital resonance (e.g. CI — CC1 2 ■*— * C1 = CC1 2 ) on the 
stability of a trihalogenomethyl anion. 55 

84 



Perfluoroalkyl Iodides 



<CF 3 ) 2 C:CF 2 + HF (C ' Hs),K > 
room temp. 




^ ( X/CtCF,,), 

(CiH s ),K loom temp. 'I / H (53%) 

(CF 3 ) 3 CH(81%)< 

CH,:CHCS 

:> (CF a ) 3 C.CHi.CH 2 .CN(64%) 



(CjH 5 ),N, 100° 



Similar use of primary or secondary fluorocarbon hydrides is complicated 
by the necessity to use much stronger bases than triethylamine to generate 
the corresponding perfluorocarbanjons. 80 Homolytic substitution of hydro- 
gen in a fluorocarbon hydride by chlorine or bromine can be effected quite 

easily, e.g., 46 

CF 3 .CF 2 .CHF 2 C \™;£T> CF 3 .CF 2 .CF 2 C1(89%) 



Bromotrifluoromethane is prepared industrially by thermal bromination of 
fluoroform at 600°. 

2. Free-radical Reactions. When a perfluoroiodoalkane is heated or irra- 
diated with u.v. light, the C — I bond undergoes homolytic cleavage to 
yield a perfluoroalkyl radical and an iodine atom, e.g., 



CF, 



CO. 250° 



or u.v. light 



> CF 3 -+I- 



The quantum yield is low, since primary recombination occurs preferentially, 
but is increased considerably in the presence of a second component that 
can react with either the radical or the atom. 

Fluorocarbon free radicals such as CF 3 - are highly reactive. They will, 
for example, abstract hydrogen from hexane, ethanol, or ether, and chlorine 
from carbon tetrachloride, and will add smoothly to olefinic bonds. Several 
examples have been given in the previous discussion of the uses in syn- 
thesis of free-radical addition reactions between olefins and perfluoroalkyl 
iodides, and many more are known. Other free-radical reactions of per- 
fluoroalkyl iodides include reduction to hydrofluorocarbons, photochemical 
conversion to fluorocarbon chlorides and bromides, direct fluorination to 
saturated fluorocarbons, and thermal interaction with metalloids to yield 
perfluoroalkyl derivatives, e.g., 

85 



Derivatives of Perfluoroalkanes 



n . C ,F,I X2 - U - V - Ught > n-C 3 F 7 X(98%) 



F, diluted with Na, 150° _ 

n-C.F 9 I ' " > n-C 4 F 10 (81%) 

4 9 An— Cu 'catalyst' 4 10v 

230° 

CF.I + P v (CF 3 ).P, <CF 3 )„PI, CF..PI, 

3 autoclave v 3 ' 3 3/2 3 2 

Photochemical oxidation of a perfluoroiodoalkane CF 3 .[CF 2 ] M .I is ex- 
tremely rapid and yields only carbonyl fluoride and, by attack on the silica 
reaction vessel, silicon tetrafluoride. Facile breakdown of a normally ex- 
tremely stable perfluoroalkyl group in this way is believed to proceed via 
formation of a perfluoroalkoxy radical CF 3 .fCF 2 ] m _ 1 .CF 2 .0-, which de- 
composes to carbonyl fluoride and the radical CF 3 .[CF 2 ] M _ 1 -, which is 
further oxidized, e.g., 58 

C 3 F,I n - T - "^ i.+CjF,. -% C 3 F 7 .0 2 - C ' F?I > C 3 F 7 .O.OI + C 3 F 7 . -^> etc. 

C 3 F 7 .O.OI U ' Y " ' 18ht > IO.+C 3 F 7 .0. — >• C 2 F 5 .+COF 3 , 

and/or C 3 F 7 .0. .iiiEi. C 3 F 7 .OI + C 3 F,. % etc. 

1 
C a F 5 .+COF 2 +I. 

C 2 F 6 . -2i C 2 F 5 .0 2 . -^i- C 2 F 6 .0.+IO.+C 3 F 7 . -^- etc. 



1 
CF 3 .+COF, 



0» (VFjI Oa 

CF 3 - -A- 0F 3 .O 2 . ' » CF 3 .0.+IO- + C 3 F 7 . — %■ etc. 



SiO a 



COF 2 + SiF 4 

In the presence of water, and particularly if chlorine or bromine is used as 
a sensitizer, the reaction intermediates undergo side-reactions to give per- 
fluoroalkanecarboxylic acids, e.g., 



n-C 3 F 7 I °" Br ',' Ha °> C 2 F 6 .C0 2 H(45%)+CF 3 .CO a H(23%) 



86 



Perfluoro-Ketones, -Thioketones, and .Ketones 

III. PERFLUORO-KETONES, -THIOKETONES, AND 
-KETENES 
A. Perfluoroketones 

The chemistry of polyfluoroketones, and particularly of commercially 
available s«/m-dichlorotetrafluoroacetone and hexafluoroacetone, has re- 
ceived detailed attention recently. 59 The following discussion deals mainly 
with hexafluoroacetone, a typical aliphatic perfluoroketone in which the 
electron-attracting perfluoroalkyl groups intensify the electrophilic pro- 
perties of the carbonyl function and thereby influence its reactivity in 
characteristic ketone reactions and also enable chemical changes not ob- 
served with hydrocarbon ketones to be effected. No tautomerism occurs 
with perfluoroketones [e.g., CF 3 .CO.CF 3 HH CF 2 :C(OF).CF 3 ] and it 
seems unlikely that it will prove possible to abstract an a-fluorine as a 
fluoronium ion (c/. p. 210) to produce a mesomeric carbanion (e. g., 
CF 3 .CO.CF 3 -II— [F]+ + [CF 2 .CO.CF 3 *^ CF 2 :C(0).CF 3 ]), so ' two im- 
portant factes of normal ketone chemistry are missing, 

1. Synthesis. Unlike their hydrocarbon counterparts, perfluoroketones can- 
not be prepared by thermal decomposition of corresponding metal carboxy- 
lates; 16 ' 80 for example, barium perfluorobutyrate, like sodium perfluoro- 
butyrate (see p. 21), yields perfluoropropene when pyrolysed, and the 
corresponding salts of trifluoroacetic acid yield mixtures of trifluoroacetyl 
fluoride and trifluoroacetic anhydride (see p. 77). Classical organometallic 
syntheses have been adapted successfully for the preparation of perfluoro- 
ketones (see ref. 1 and pp. 106, 108), but in general the methods are tedious 
and yields are not good. 

Commercially, 59 C ' S1 hexafluoroacetone is prepared by halogen exchange 
between perchloroacetone and hydrogen fluoride in the presence of a di- 
chromium trioxide catalyst; it can also be prepared by permanganate 
oxidation of perfluoroisobutene (see p. 35), readily obtained in the labora- 
tory by pyrolysis of polytetrafluoroethylene (see p. 21), and by isomeriza- 
tion of the epoxide of perfluoropropene (see p. 163) under the influence of 
Lewis acid catalysts, e.g., 62 

CF 3 .FC-^-CF 2 ^-> CF 3 .CO.CF 3 (83%) 

The epoxide method can also be used to obtain higher homologues of 
hexafluoroacetone, starting materials being prepared by reaction of the 
corresponding perfluoro-olefins with aqueous alkaline hydrogen peroxide, 



e.g <* 



^Os 



CF,[CF 2 ],CF:CF 2 - ^* > CF 3 .[CF 2 ],FC^CF 2 ^ CF..COTJ..CO.CF, 



87 



Berivabveea of Perflktoroalkanes 

Another method, leading to symmetrical higher homologues, involves 
treatment of the ethyl ester of a perfluoroalkanecarboxylic acid with a 
0-5 molar proportion of a sodium alkoxide, followed by acidification of the 
product and dehydration of the perfluoro-s«/m-ketone hydrate (see later) 
subsequently isolated, e.g., 64 

1. ether, 25° 
2n-C 3 F 7 .C0 2 C 2 H 5 + lC 2 H 5 .ONa g Ha80< > 

(C 2 H 5 .0) a CO + (n-C 3 F,) 2 C(OH) 2 -^^* (n-C 3 F,) 2 CO(88%) 

Esters of perfluorinated acids are known to yield 1 : 1 adducts with sodium 
alkoxides, 65 so presumably reactions of the above type proceed as follows : 

,0 O") Na + f O 

B F — C— O.B — > B.O— C^f" C— Bp — ► (B.O) 2 CO + (Bf) 2 C\ 
B.O~Na + B.O O.B H s S0 4 



(Bf=C 2 F 5 , CsFj.e'c.; B=C 2 H5) 



(BF^CCOH^B.OH+NaHSO, 



Reaction of perfluoroacyl fluorides with perfluorocarbanions generated 
in situ from perfluoro-olefins and an alkali-metal fluoride in acetonitrile can 
be employed to prepare either symmetrical or unsymmetrical perfluoro- 
ketones, as shown earlier (p. 33) ; symmetrical ketones can be obtained 
more directly using carbonyl fluoride and an excess of perfluoro-olefin: 86 



R' COF 
— >- (Kf)(CF 8 )CF.CO.E f +F- 



B r .CF:CF 2 +F- ^ (R F )(CF 3 )CF-— ' 

\ COF» 



> (B F )(CF 8 )CF.COF+F- 



(Rf)(CP,)CF- 

(B P )(CF s )CF.CO.CF(CF 3 )(R F ) +F" 
(R F = F or perfluoroalkyl; R F = perfluoroalkyl) 

Note that perfluoroacyl fluorides can be obtained simply and quantitatively 
by reaction of the corresponding chlorides with anhydrous sodium fluoride 
in acetonitrile at room temperature; 67 the same type of method can be 
used to convert commercial phosgene into carbonyl fluoride. 68 

Use of dicarboxylic acid fluorides, FOC.tCF^.COF (x = 0, 1, 2, 3, etc.), 
in the last method enables perfluorinated diketones, Rj..CO.[CF 2 ], I ..CO.R F 
(Rj. = perfluoroalkyl derived from a perfluoro-olefin and fluoride ion), to 
be synthesised. 66 When oxalyl fluoride (x = 0) and perfluoropropene are 
employed in this reaction, the product is the perfluoro-oc,/?-diketone 

88 



Perfluoro-Ketones, -Thiohetones, and -Ketenes 

(CF 3 ) a CF.CO.CO.CF(CF 3 ) 2 ; other members of this class can be synthesised 
by pyrolysis of enediol di-esters prepared by condensation of perfluoroacyl 
chlorides in the presence of nickel carbonyl 69 and by treatment of 2,3-di- 
chlorohexafluorobut-2-ene with chromium trioxide in fuming sulphuric acid 
(to yield perfluorobiacetyl). 70 Perfluorocyclobutan-l,2-dione can be prepared 
by reaction of 95% sulphuric acid with 1,2-dimethyloxyhexafluorocyclo- 
butane, which is obtained in standard fashion (see p. 46) by thermal 
dimerization of methyl perfluorovinyl ether. 71 An analogous method can 
be used to prepare perfluorocyclobutanone (see p. 166), and perfluorocyclo- 
pentanone and perfluorocyclohexanone can also be produced by treatment 
of the corresponding methyl perfluorocycloalkyl ethers with sulphuric acid : ,2 



[OTJ. 



CF 



.CF 



CHj.OH, 
KOH pellets 




O.CH, 



[OTil 



N 2 , CoF 3 



90-150" 




CF.O.CH, 



95% H 2 S0 4 
175-190° 




[CF a l 



(OH), 



P>0 5 




2. Reactions of Hexafluoroacetone. Hexafluoroacetone is a colourless, 
hygroscopic, non-flammable, toxic gas (b.p. -27-5°; cf. acetone, b.p. 56°) 
that is stable at 300° but decomposes at higher temperatures to yield hexa- 
fluoroethane and carbon monoxide. These two products are formed almost 
exclusively when hexafluoroacetone is subjected to u.v. irradiation, the 
ketone being a 'clean' photochemical source of trifluoromethyl radicals : 



u.v. light 



[CF 3 .CO.CF 3 ]* 



-* CO+2CF, 



->• C,F. 



As indicated in the above scheme, hexafluoroacetone yields two trifluoro- 
methyl radicals and carbon monoxide as the principal products of primary 
photodissociation, 69 °- 73 whereas acetone yields a methyl and an acetyl radical. 
The dipole moment of hexafluoroacetone is much lower than that of 
acetone (2-26 D vs. 2-75D) owing to the influence of the two highly electro- 
negative CF 3 groups on the polarity of the carbonyl bond. This inductive 
influence reduces the basicity of the carbonyl oxygen to such an extent 
that no evidence can be found by n.m.r. techniques for protonation of 



89 




Derivatives of Perfluoroalkanes 

hexafluoroacetone in the strongly acidic solvent system FS0 8 H — SbF 8 — 
— S0 2 ; 74 by contrast, 1,1,1-trifluoroacetone is quantitatively protonated by 
' this solvent system at — 60°, and acetone is almost completely protonated 
even by concentrated sulphuric acid. The great resistance shown by hexa- 
fluoroacetone towards attack by phosphorus pentachloride presumably also 
stems from lack of electron density at the carbonyl oxygen (CF 3 .CC1 2 .CF 3 
forms at a reasonable rate only at temperatures above 250°) ; 76 however, 
aluminium chloride appears to co-ordinate satisfactorily to the carbonyl 
oxygen since it promotes reaction between hexafluoroacetone and benzene 
or its homologues under mild conditions, whereas toluene-p-sulphonic acid 
is an ineffective catalyst even at 300° : " 

Aid, Ij^N— C(CF 3 ) 2 .OH(94%) 
+ CF 3 .CO.CF 3 —^> 

Monohalogenobenzenes also suffer electrophilic attack by hexafluoroacetone 
in the presence of aluminium chloride, e.g., 77 



(CF,),C^0-X1C1 3 .. . 

60°, autoclave ' II I < 66 /o > 



and so does ethylene, 698 but isobutene, a more nucleophilic olefin, reacts 
with hexafluoroacetone at room temperature in the absence of an acti- 
vator: S9a,e 

(CFsJjC 1 — =0 go" 

*\ v -» r — »■ (CF 3 ) ii C(OH).CH 2 .C(:CH !! ).CH3 

C^CHa 

I 
CH„ _ 

As indicated by the last result, even weakly nucleophilic reagents readily 
attack the electron-deficient carbonyl carbon atom in hexafluoroacetone. 
Much attention has been given to this aspect of the chemistry of hexa- 
fluoroacetone and, as with hydrocarbon ketones, quite a lot is known about 
reactions involving hetero-atom nucleophiles, active methylene compounds, 
and organometallic reagents. 69 

In general, 78 direct addition of hetero-atom nucleophiles to a ketonic 
carbonyl function leads to an isolable product only if followed by an exo- 
thermic dehydration along one of three routes : 

90 





Perfluoro- Ketones, -Thioketones, and -Ketones 



HA, H+ I I 

>■ — C— C— A + H 2 



I I 
— C— C=0 + HA 

I 
(H) 



— C— C— OH- 

! I 

(H) A 



(e.g., ketal formation) 

-> \>=o/ + H a O 
A 
(e.g., a enamine formation) 



»■ — C— C=A' + H 2 

I 

(e.g., "oxime, phenylhy- 
drazone, semicarbazone, 
or Schiff's base formation) 

(" The ketone must carry an a-hydrogen. b The attacking hetero atom must 
carry a hydrogen substituent.) 

Perfluoroketones are exceptional since they yield isolable addition products 
with oxygen, nitrogen, sulphur and halogen nucleophiles owing to the 
favourable position of the equilibrium 

(R p ) a C:0+HA v (BjJ^OHJ.A 

[R p = perfluoroalkyl; A = OH, OR, NH 2 , NHR, SH, F (R = alkyl)] 

which, in the absence of de-stabilising steric effects in the adduct, lies far 
to the right at room temperature, in contrast to the opposite situation with 
hydrocarbon aliphatic ketones. 

Thus an exothermic reaction occurs when hexafluoroacetone is treated 
with one molar proportion of water to give a solid monohydrate, 
(CF 8 ) a C(OH) a , m.p. 49°, which reverts to the ketone when treated with 
concentrated sulphuric acid or phosphorus pentoxide. Dissolution of this 
grem-diol in half its molar equivalent of water yields a liquid sesquihydrate. 
The monohydrate is acidic [?K a 6-58 (H 2 0, 25°); cf. CH 3 .C0 2 H, 4-76], a 
property derived from the inductive influence of the two trifluoromethyl 
groups and probably enhanced by stabilization of the mono-anion by 
hydrogen bonding as in (V) or (VI).' 9 Both the monohydrate and the sesqui- 
hydrate form strong hydrogen bonds with basic acceptors (e.g., amines, 
ethers), and the liquid sesquihydrate and higher hydrates are excellent 
solvents for polymers with receptive sites [e.g., nylons, polyacrylonitrile, 
polyacetals, poly(vinyl alcohol), proteins]. 



(CFa)jC\. /. 
x O x 
(V) 



H 



/° H \ 
(CF S ) 2 C< >0-H 

x O— H 
(VI) 



Provided no undue amount of steric strain is imposed through branching 
in the alkyl groups, primary and secondary alcohols combine with hexa- 



91 



Derivatives of Perfluoroalkanes 

fluoroacetone to yield hemi-ketals, which require treatment with an alkylat- 
ing agent to provide ketals, e.g., 

(CF 3 ) 2 C:0+C 2 H 5 .OH > (CF 3 ) 2 C(OH).O.C 2 H, _ (CH3> ' S0 " K ' co %. 

or CHjN 2 

(CF 3 ) 2 C(O.CH 3 ).O.C 2 H 5 

In marked contrast to their hydrocarbon analogues these ketals resist 
hydrolysis by acids even under forcing conditions, 80 presumably owing to 

the difficulty of gaining access to the highly unstable carbonium ion inter- 

+ 
mediates of type (CF 3 ) 2 C.OR (It = alkyl) because the powerful inductive 
effect of two trifluoromethyl groups militates against both protonation of 
an adjacent oxygen atom and the required heterolytic carbon-oxygen 
fission should protonation occur. 

Isolable adducts of type (CF 3 ) 2 C(OH).N< can be prepared by treating 
hexafluoroacetone with ammonia, aliphatic or aromatic amines, hydrazine, 
amino acids, amides, hydroxylamine, and semicarbazide, and the com- 
pounds (CF 3 ) 2 C(OH).N 3 and (CF 3 ) 2 C(OH).NCO can be synthesised using 
hydrozoic acid and cyanic acid, respectively. The adducts derived from 
ammonia and amines can be dehydrated to imines (Schiff bases), e.g., 

(CF 3 ) 2 C:0 in pyridine h C " 3 "™ a ' ~ ~ **> (CF 3 ) 2 C:N.CH 3 (87%) 
2. POCI3 (exothermic) 3 

but methods for the dehydration of other types of adduct have not been 
developed. However, several classical ketone derivatives have been prepared 
from the imine of hexafluoroacetone (see p. 145) or its .W-phenyl derivative, 

e-gv 

(CF 3 ) 2 C:0+C 6 H 5 .NCO (0 ' S ^ :0 < cata f tic amonnt> > 

(CF 3 ) 2 C:N.C 6 H 5 (92%)+(C a H 5 ) 3 P:0 
NH 2 OH _^ (CF 3 ) 2 C(NH.C 8 H 5 ).NHOH -i^ 



/ ethanol 

(CF 3 ) 2 C:N.C e H 5 — { (CF 3 ) 2 C:N.OH + 6 H 6 .NH 2 

\NH 2 .NH.CO.NH s „ 160° 

ethanol (CF 3 ) 2 C(NH.C 6 H 5 ).NH.NH.CO.NH 2 > 

(CF 3 ) 2 C:N.NH.CO.NH 2 +C 6 H 5 .NH 2 

In the preparation of the ^-phenyl imine, shown above, the small amount 
of triphenylphosphine oxide combines initially with phenyl isocyanate to 
give the phosphine imine (C s H 5 ) 3 P:N.C 6 Hs, which then reacts with hexa- 
fluoroacetone to give the required product and regenerate the phosphine 
oxide. Similarly, Wittig reagents, even quite stable ones that react reluc- 
tantly or not at all with acetone, attack the highly electrophilic carbonyl 
group in hexafluoroacetone to give olefins containing the (CF S ) 2 C : C< group ; 
this is a useful conversion since compounds of type (CF 3 ) 2 C(OH).CHR 2 , 

92 



Perfhwro-Ketones, -Thioketones, and -Ketenes 

although easily obtained, are difficult to dehydrate to olefins (c/. the resist- 
ance of bistrifluoromethyl ketals to hydrolysis, p. 92). 

Treatment of hexafluoroaeetone with potassium or caesium fluoride in 
an aprotic solvent yields the corresponding heptafluoroisopropoxides, 
(CF 3 ) 2 CF.O-M+ (M = K or Cs; see p. 168); the parent alcohol, heptafluoro- 
isopropanol, can be obtained from hexafluoroaeetone and an equimolar 
proportion of hydrogen fluoride at 0°, but it is unstable and has not yet 
been isolated in the pure state (see p. 166). Hydrogen cyanide combines 
readily with hexafluoroaeetone in the presence of a small amount of piper- 
idine to give the isolable cyanohydrin (CF 3 ) 2 C(OH).CN, which can also be 
prepared by acidification of its sodium salt, obtained from the ketone and 
sodium cyanide in acetonitrile. 

Typical Knoevenagel condensation catalysts promote reaction between 
hexafluoroaeetone and active methylene compounds, but in contrast to 
common experience with hydrocarbon ketones, the initial adducts do not 
lose water readily to give olefins, e.g., 

(CF 3 ) 2 0:O +CH 2 (CO a C 2 H 5 ) a py ^ ne , (CF 3 ) 2 C(OH).CH(C0 2 C 2 H 5 ) 2 (79%) 
(CF 3 ) 2 C : 0+CH 2 (CN) 2 -^V (CF 3 ) 2 C(OH).CH(CN) 2 (100%) P2 ° $ 



80 o- i-,,,-^-,.—^,.!— », heatstronBly' 

(CF S ) 2 C:C(CN) 2 (50%) 

Bistrifluoromethylearbinols can also be prepared from hexafluoroaeetone 
and organometallic reagents. In the absence of steric effects, Grignard 
reagents, for example, readily attack hexafluoroaeetone, the difference in 
reactivity between this ketone and acetone being revealed by the observa- 
tion that when a mixture of the two is treated with a limited amount of 
phenylmagnesium bromide and then acidified only the carbinol 
(CF 3 ) 2 C(OH).C 6 H 6 is formed. In keeping with experience in hydrocarbon 
chemistry, the branched Grignard reagent isopropylmagnesium bromide 
converts hexafluoroaeetone into 2£f-hexafluoroisopropanol (after acidifica- 
tion) in 94% yield: 

(CF 3 ) 2 C=0 

hJ Mg— Br — »• (CF 3 ) 2 CH.O.MgBr+CH 2 :CH.CH 8 

\ f~/ 
HjC— CH 

I 
CH 3 

Hexafluoroisopropanol is best prepared by conventional reduction of 
hexafluoroaeetone with hydrogen in the presence of platinum or copper/ 
chromium oxide catalysts or with lithium aluminium hydride; 89 reduction 
with magnesium amalgam or, in higher yield, by a photochemical tech- 
nique (see p. 167) gives perfluoropinacol, an extremely toxic, strongly 

93 



Derivatives of Perflnoroalkanes 

acidic diol [pK a 5-95 (H s 0, 25°)].'» Perfluoropinacol does not rearrange 
to perfluoropinacolone even on prolonged treatment with hot 100% 
sulphuric acid; 81 presumably this reflects the difficulty of effecting for- 
mation of an incipient carbonium ion of type (CF 3 ) 2 C— from the corre- 
sponding alcohol. 

Free radicals will abstract a-hydrogen from a hydrocarbon ketone but 
will not, in general, attack the carbonyl group; by contrast, hexafluoro- 
acetone with its weakly polarized electron-deficient carbonyl function 
readily participates in radical-addition reactions, 82 e.g., 

./\/C(CF a ) 2 .OH 
i //-,-r, \ ~ ~ benzoyl peroxide f i 

+ (0F,) 2 C : O -£ > I ( 57 %) + 




80° 



/ O.CH(CP 3 ) 2 

(3%) 



Finally, hexafluoroacetone can be cleaved in haloform fashion with 
aqueous base, and acts as a potent dienophile towards electron-rich 1,3- 
dienes in the Diels-Alder reaction, e.g., 83 

Off o 

CH S .C^ * C(CF 3 ) 2 l0()o CH S .C/ C ^C(CF 3 ) 2 

I + I 8 ealedtube > I I < 92% > 

CH8 - C ^CH 2 ° CH 3- C \ c /° 

B. Perfluorothioketones 

Several routes to perfluorothioketones have been disclosed. 84 The pre- 
ferred methods appear to be reaction of a perfluoro-s-alkyl iodide with 
phosphorus pentasulphide, e.g., 

CF 3 .CFI.C a F 5 -££* CF„.C(:S).C 2 F 6 (92%) 

and reaction of a bis(perfluoro-s-alkyl) mercurial with boiling sulphur, e.g., 

445° 

[(CF 3 ) 2 CF] 2 Hg+2S >■ 2(CF 3 ) 2 C:S(60%) + HgF 2 

Both types of starting material can be prepared from perfluoro-olefins (see 
pp. 81, 111); use of precursors containing primary perfluoroalkyl groups 
enables thio-acid fluorides to be prepared, 84 e.g., C 2 F 5 I/P 2 S 5 or (C 2 F 5 ) 2 Hg/S 
gives trifluorothioacetyl fluoride, CF 3 .C(:S)F. However, hexafluorothio- 
acetone is best prepared directly from perfluoropropene by mixing it with 
sulphur vapour and passing the mixture through a bed of active carbon 

M 



Perfluoro-Ketones, -Thioketones, and -Ketenes 

at 425°; application of this method to other perfluoro-olefins gives only 
poor yields of pernuorinated thiocarbonyl compounds. 85 

Knowledge of perfluorothioketones mainly concerns hexafluorothio- 
acetone, which, like all monomelic thiones, 8 * is coloured. It is a deep blue 
compound, b.p. 8°, which readily dimerizes to a 1,3-dithietane at room 
temperature (c/. 86 thioacetone, which rapidly cyclo-trimerizes) and must 
be stored at low temperatures to arrest this change; pyrolysis of the colour- 
less dimer regenerates the monomer: 

FsC\ 25° F,C X /S v /CF, 

2 >c=s:^=± >C< >C< 
FsCT «00° FjCT N S X N CF 3 

Unlike hydrocarbon thioketones hexafluorothioacetone is not attacked by 
atmospheric oxygen, and, in contrast to hexafluoroacetone (see p. 91), it 
does not react with water in the absence of a catalyst. 8 * However, like 
hexafluoroacetone, only in more reactive fashion, it does combine with 
olefins containing allylic hydrogen, 87 e.g., 

(CF,) 2 C==S 

*-> /*• ~ 78 ° ' C 

H /^(CH,)* -r->" .(CF 3 ) 2 CH.S.C(CH 8 )2.C(CH,):CH 2 (72%) 

H„p— C 

CH 3 

and with electron-rich 1,3-dienes, 88 e.g., 

H 2 

CH C 

HC-^ 2 C(CF 3 ) 2 , ao HC-^ ^C(CF 3 ) 2 

| + | "^ I! | O0%) 

HC ^CH S HC \ C / S 

H 2 

but the products from the former type of reaction are allyl sulphides and 
not unsaturated mercaptans [e.g., (CP 3 ) 2 C(SH).C(CH3) 2 .C(CH S ):CH 2 from 
the above reaction with tetramethylethylene] as would be expected by 
analogy with hexafluoroacetone (see p. 90). Hexafluorothioacetone is also 
highly susceptible to attack by more conventional nucleophiles, and ex- 
amples are known 89 of both normal and reverse addition across the thio- 
carbonyl link, e.g., 

78° 

(CF 3 ) a C:S+CH 3 .SH > (CF 3 ) a C(S.CH,).SH 

(CF 3 ) 2 C: S + NaHSO, -^£U (CF S ) 2 CH. S. SO s Na 

Study of this interesting phenomenon has been hampered by the fact that 
bases catalyse the dimerization of the thioketone. 

95 



Derivatives of Perfluoroalkanes 



C. Perfluoroketenes 

Knowledge in this area of fluorocarbon chemistry has been gained almost 
entirely through studies on bistrifluoromethylketene, 90 which was first syn- 
thesised in the early 1960's. This ketene is isolable, unlike the other per- 
fluoroketenes claimed in the literature, viz., difluoroketene and fluorotri- 
fluoromethylketene, which are not and thus resemble dichloro- and di- 
bromo-ketene. 91 Bistrifluoromethylthioketene appears to be the only per- 
fluorotbioketene reported so far. 92 

1. Transient fluoroketenes. The claim 93 that difluoroketene can be pre- 
pared by application of a classical Staudinger method (dehalogenation of 
an a-halogenoacyl halide with zinc) to chlorodifluoroacetyl bromide has been 
disputed, 94 ' 95 but it does appear possible to generate this ketene from 
bromodifluoroacetyl chloride or bromide and amalgamated zinc: 96 

,™ ^ CHa.ONa, 60°/40p.s.i. Br» 

CF a =CF 2 f ^ wj > CF 2 :CF.O.CH 3 (52%) ** - 



tetrahydrofuran " - •» — " ' < _ 300 

CF s Br.CFBr.O.CH 3 (80%) _ ^f^. > CF 2 Br.COCl(58%) + CF 2 Br.COBr(l5%) 

Zn\ <0° /Zn 




CF 2 :0:O 



Difluoroketene is short-lived, even at - 5°, and proof of its formation as 
shown above rests mainly on the isolation of cyclo-adducts when bromo- 
difluoroacetyl chloride is dehalogenated in the presence of acetone or benzal- 
dehyde and of carbon monoxide and tetrafluoroethylene in experiments 
at 35°: 



CFjBr.COCl 




Zn, acetone 



F 2 C=C=0 
(CH 3 ) 2 C=0 



F 2 C— C=0 



(CH 3 ) 2 C— O 



(50%) 



Zn, ether 



35° 



> 0F 2 :C:O 



-> CO + 



:CF. 
:CF 2 >■ CF 2 :CF 2 



Attempts to generate difluoroketene by dehydrohalogenation of difluoro- 
acetyl chloride with triethylamine (i.e., by the application of another 
classical Staudinger method) appear to have failed, 96 ' 97 but monofluoro-, 958 
chlorofluoro-, 9Bb and fluorotrifluoromethyl-ketene 9Sb do seem capable of 
generation in this way, as indicated by the results of trapping experiments 
involving cyclopentadiene : 

— /B 

^F + (CjjH s )aNHCr 



R.CHF.COC1 + 



<C«H,),N 



^O 



(R = H 40%; B = 01, 12%; B = CF 3 , 60%) 



96 



Perfluoro-Ketones, -Thioketones, and -Ketones 

Difluoroketene dimethyl acetal can be prepared from trifluoropyruvyl 
fluoride, 9 * which is formed 99 when perfluoropropene epoxide (see p. 163) is 
heated with benzophenone : 

CF3.CO.COF CH,.Q W .,CH,.OH 

" - 80 to 25° 

1. CH,.ONa, CH..OH 

CP I .C(OH)(O.CH s ).CO a CH s (88%) ^1 ► 

8 2. (CH,),S04 

beat 

CF,C(O.CH a ) 2 .CO,CH, K0H>0 ^; 0Haq > CF,C(O.CH a ) 2 .C0 2 K . m ^°°\ 

CF 2 :C(O.CH,) a (99%)+CO a +KF 

2. Bistriflaoromethylketene. 

(CF s ) a C:CF a *'°;"° > £(CF,) 2 CH.CF..OH] — > 

* a s tetrahydrofuran lv " 2 2 J (see p. 165) 

[(CF 3 ) 2 CH.COF] -^v <CF 3 ) 2 CH.CO a H -|g* (CF 3 ) a C:CiO<94%) 
(m.p. 49-50°) 

This ketene, b.p. 5° [c/. (CH 3 ) 2 C:C:0, b.p. 34°], which can be prepared 
quite easily from perfluoroisobutene as shown above, 100 is the main subject 
of a recent review. 90 Briefly, it neither reacts with atmospheric oxygen nor 
dimerizes spontaneously to a tetra-substituted cyclobutan-l,3-dione, in 
contrast to its hydrocarbon counterpart which does both; in fact it is 
stable for long periods at 250° under neutral conditions and is unaffected 
by oxygen at 200° under pressure." However, it does react readily and in 
typical ketene fashion with nucleophilic reagents, e.g., 

H s O 
> (CF,) 2 CH.C0 2 H 



(CF 3 ) a C:C:0- 



BOH 
(B ■» alkyl or aryl) v *'* 2 



— > {CF 3 ) 2 CH.c6.NH a 



BB/NH 

-> (CF s ) 2 CH.CO.NRR' 



(B - H or alkyl; B' - alkyl or aryl) 



HX 
(X - CI, Br. O.CO.CH,, O.CO.CF,) * < CF »>2CH.COX 



97 



Derivatives of Perfltioroalkanes 

Fluoride ion also attacks the electrophilic carbonyl carbon atom in bistri- 
fluoromethylketene, and this forms the basis of the vapour-phase iso- 
merization of the ketene to perfluoro(methacryloyl) fluoride : 

NaF, 800°/l atm. 

(CF g ) a C:C:0 — -^ ' ■ > CF a :C(CF.).COF(34%)+recovered ketene (63%) 

flow method 

and of its liquid-phase dimerization to a /S-lactone : 

,„„ . „ „ „ CbF (catalytic amount) (CF 8 ) a C— C:0 

(CFs),C:C— O 

These conversions can be rationalised in terms of carbanion intermedi- 
ates. 100 ' 101 Under the conditions used to isomerize bistrifluoromethylketene, 
an equilibrium is established which favours the ketene rather than the 
acryloyl fluoride and thus reflects the greater stability of the cumulene 
system with its internal C:C bond. 

Cyclo-addition reactions involving ketenic C : C bonds and the unsaturated 
sites in olefins, aldehydes, etc., form an important aspect of ketene chem- 
istry 102 in which bistrifluoromethylketene, 90 ' 103 e.g., 

(0F,),C=C:O . , . . , (CF 8 ) 2 C C:0 

trace of hydroquinane ""i i 

„„ 100°, sealed tube * I I ( 80% ) 

C,Hs-CH=CH 2 C 6 H 5 .CH-CH 2 

and the transient ketenes CF 8 :C:0 and CF 3 .CF:C:0 (see p. 96) also 
partake. 

REFERENCES 

1. For more detailed accounts consult Lovelace, Rausch, and Posteln-ek, 
Aliphatic Fluorine Compounds, Reinhold, New York, 1958; Httdlicky, Chem- 
istry of Organic Fluorine Compounds, Pergamon, London, 1961; Simons and 
Bbice, Fluorine Chemistry, ed. Simons, Academic Press, New York, 1954, 
Vol. II, p. 333; and The Kirk-Othmer Encyclopedia of Chemical Technology, 
2nd edition, Interscience, New York, 1966, Vol. 9, p. 686. 

2. Henne and Trott, J. Amer. Chem. Soc, 1947, 69, 1820. 

3. McBbb, Wiseman, and Bachman, Ind. Eng. Chem., 1947, 89, 415. 

4. TSxraiAND, Listdsey, and Meiby, J. Amer. Chem. Soc, 1958, 80, 6442. 

5. Inman, Obstebling, and Tyczkowski, J. Amer. Chem. Soc., 1958, 80, 6533. 

6. Haszeldine, J. Chem. Soc., 1951, 588; ibid., 1952, 2504; Haszemjijtb and 
Leedham, ibid., 1954, 1634. 

7. Haszeldine and Leedhah, J. Chem. Soc, 1953, 1548. 

8. Hood, Reduch, and Reilly, J. Chem.. Phys., 1955, 28, 2229. 

9. Htmas and Gabber, J. Amer. Chem. Soc, 1959, 81, 1847. 

10. Hxtsted and Abxbbecht, J. Amer. Chem. Soc, 1952, 74, 5422. 

11. Hbnne and Fbancis, J. Amer. Cht?m. Soc,, 1953, 75, 991. 

98 



References 

12. Krogh, Reid, and Brown, J. Org. Chem., 1954, 19, 1124; Fields and Has- 
zeldine, J. Chem. Soc. 1964, 1881. 

13. Haszeldine, J. Chem. Soc., 1963, 2075. 

14. Barb and Haszeldine, J. Chem. Soc., 1956, 3416. 

15. Lagowski, Quart. Rev., 1959, 13, 233. 

16. LaZerte, Hals, Beid, and Smith, J. Amer. Chem. Soc, 1953, 75, 4525. 

17. Kbooh, Beid, and Bbown, J. Org. Chem., 1954, 19, 1124. 

18. Barb and Haszeldine, J. Chem. Soc, 1957, 30. 

19. Babb and Haszeldine, J. Chem. Soc, 1956, 3428. 

. 20. For a review see Knunyants, Chih-Yuan, and Shokina, Russ. Chem. Rev., 
1963, 82, 461. 

21. G.P. 1,231,679/1965 (Chem. Abs., 1967, 66, 55057 s, p. 5184). See also Swabts, 
Bull. eci. aead. roy. Belg., 1931, 17, 27 and Conway and Dzebciuoh, Gonad. 
J. Chem., 1963, 41, 38 for reports on the electrolysis of trifluoroacetic aoid, 
and Levin, Chechina, and Sokolov, J. Cten. Chem. V.S.S.R., 1965, 85, 1776 
for an account of the application of the Kolbe reaction to coH-polyfluoro- 
alkanecarboxylic acids. 

22. Musgbave, Quart. Rev., 1954, 8, 331; Tedder, Chem. Rev., 1955, 55, 787. 

23. Blake and PritcharD, J. Chem. Soc. (B), 1967, 282. 

24. Swabts, Bull. sci. acad. roy. Belg., 1898, 85, 375. 

25. See, for example, Birchall, Cboss, and Haszeldine, Proc. Chem. Soc, Land., 
1960, 81 ; Knox, Velabde, Berger, Cuadbiello, Landis, and Cross, J. Amer. 
Chem. Soc, 1963, 85, 1851; Kirmse, Carbene Chemistry, Academic Press, 
New York, 1964; Herkes and Burton, J. Org. Chem., 1967, 82, 1311 ; Popper, 
Cablon, Mabigliano, and Yudis, Chem. Gomm., 1968, 277. 

26. Weygand and Scendes, Angew. Chem., 1952, 64, 136; Wolfrom and Bhat, 
J. Org. Chem., 1967, 82, 1821. 

27. Clembnti, Genel, and Marino, Chem. Coram., 1967, 498. 

28. Emmons, J. Amer. Chem. Soc, 1954, 76, 3468. 

29. Emmons and Pagano, J. Amer, Chem. Soc, 1955, 77, 4557. 

30. Emmons, J. Amer. Chem. Soc, 1954, 76, 3470. 

31. Emmons and Pagano, J. Amer. Chem. Soc, 1955, 77, 89. 

32. Emmons, Pagano, and Freeman, J. Amer. Chem. Soc, 1954, 76, 3472. 

33. Emmons and Lucas, J. Amer. Chem. Soc, 1955, 77, 2287. 

34. Chambers, Goggin, and Musgbave, J. Chem. Soc, 1959, 1804. 

35. Habt and Bubhleb, J. Org. Chem., 1964, 29, 2397; Habt, Bothies, Waring, 
and Meyeeson, ibid., 1965, 30, 331. 

36. Habt and Lbbner, J. Org. Chem., 1967, 32, 2669. 

37. Pabtch, J. Amer. Chem. Soc, 1967, 89, 3662. 

38. Banes, Emeleus, Haszeldine, and Kbbbigan, J. Chem. Soc, 1948, 2188; 
Emeleus and Haszeldine, ibid., 1949, 2948. 

39. Henne and Finnegas, J. Amer. Chem. Soc, 1950, 72, 3806. 

40. Haszeldine, Nature, 1950, 166, 192; J. Chem. Soc, 1951, 584. 

41. See Haszeldine, Fluorocarbon Derivatives, Royal Institute of Chemistry (Lon- 
don) Monograph, 1956, No. 1; J. Chem. Soc, 1954, 4026; and Hudlicky, 
Chemistry of Organic Fluorine Compounds, Pergamon, London, 1961. 

42. Haszeldine and Shabpe, J. Chem. Soc, 1952, 993. 

43. Kbespan, J. Org. Chem., 1962, 27, 1813. 

44. Chambers, Musgbave, and Savory, J. Chem,. Soc, 1961, 3779; Hauptschein 
and Braid, J. Amer. Chem. Soc, 1961, 88, 2383. 

45. Young and Reed, J. Org. Chem., 1967, 82, 1682. 

46. Haszeldine, J. Chem. Soc, 1953, 3761. 

47. Banus, Emeleus, and Haszeldine, J. Chem. Soc, 1951, 60. 

99 



Derivatives of Perflnoroalkan.es 

48. Haszeldine, J. Chem. Soc, 1953, 2622. 

49. Henne, J. Amer. Chem. Soc, 1950, 72, 299; LaZebte, Hals, Reid, and Smith, 
J. Amer. Ohem. Soc, 1953, 75, 4525. 

50. Andbeades, J. Amer. Chem. Soc, 1964, 86, 2003. 

51. Cram, Fundamentals of Garbanion Chemistry, Academic Press, New York, 1965. 

52. Campbell, Stephens, and Tatlow, Tetrahedron, 1965, 21, 2997. 

53. Stbbitwieseb and Holtz, J. Amer. Chem. Soc, 1967, 89, 692. 

54. Stbbitwieseb, Mabohand, and Ptojaatmaka, J. Amer. Chem. Soc, 1967 89 
693. 

55. Hine, Btjbske, Hine, and Langfobd, J. Amer. Chem. Soc, 1957, 79, 1406. 

56. Knttnyants, Chebubkov, Babqamova, and Rokhlin, JDokl. Akad. Nauk 
S.S.S.R., 1965, 165, 827. 

57. Knunyants, Kochabyan, and Rokhi.es, Izvest. Akad. Nauk. S.S.S.B., Ser. 
Khim., 1965, 1910; Kochabyan and Kolomnikova, ibid., 1966, 1288; Kocha- 
byan, Rokhlin, Chebubkov, and KsrarANis, ibid., p. 1870. 

58. Fbancis and Haszeldine, J. Chem. Soc, 1955, 2151. 

59. For reviews see: (a) Gambabyan, Rokhlin, Zeifman, Ching-Yttn, and 
Knunyants, Angew. Chem. (International Edition), 1966, 6, 947; (b) Woolf 
in The Kirk Othmer Encyclopedia of Chemical Technology, 2nd edition, Inter- 
science, New York, 1966, Vol. 9, p. 754; and (c) Kbespan and Middleton in 
Fluorine Chemistry Reviews, ed. Tarrant, Arnold (London) and Dekker (New 
York), 1967, Vol. 1, p. 145. 

60. Simons, Bond, and McAbthub, J. Amer. Chem,. Soc, 1940, 62, 3477 

61. B.P. 1,038,296/1966. 

62. Moobe and Milian, B.P. 1,019,788/1966; XJ.S.P. 3,321,515/1967. 

63. Mobin, XJ.S.P. 3,213,134/1965. 

64. Wiley, XJ.S.P. 3,091,643/1963. 

65. Swabts, Bull. Soc. chim. beiges, 1926, 85, 412; Bendeb, J. Amer. Chem. Soc, 
1953, 75, 5986. 

66. Smith, Fawcett, and Coffman, J. Amer. Chem. Soc, 1962, 84, 4285. 

67. See, for example, Redwood and Willis, Canad. J. Chem., 1967, 45, 389. 

68. Tullook and Coffman, J. Org. Chem., 1960, 25, 2016; Fawcett, Tdxlock, 
and Coffman, J. Amer. Chem. Soc, 1962, 84, 4275. 

69. Dbysdale and Coffman, J. Amer. Chem. Soc, 1960, 82, 511. 

70. Moobe and Clabk, J. Org. Chem., 1965, 30, 2472. 

71. England, J. Amer. Chem. Soc, 1961, 88, 2205. 

72. Clayton, Roylance, Saybbs, Stephens, and Tatlow, J. Chem. Soc, 1965, 
7358; Clayton, Stephens, and Tatlow, ibid., p. 7370. 

73. McIntosh and Pobteb, Trans. Faraday Soc, 1968, 64, 119. 

74. Olah and Pittman, J. Amer. Chem. Soc, 1966, 88, 3310. 

75. Fabah and Gilbebt, J. Org. Chem., 1965, 80, 1241. 

76. Fabah, Gilbebt, and Sibika, J. Org. Chem., 1965, 30, 998. 

77. Sheppabd, J. Amer. Chem. Soc, 1965, 87, 2410. 

78. Gtjtsohe, The Chemistry of Garbonyl Compounds, Prentice Hall, Inc., Engle- 
wood Cliffs, N. J., 1967. 

79. Middleton and Lindsey, J. Amer. Chem, Soc, 1964, 86, 4948. 

80. Simmons and Wiley, J. Amer. Chem. Soc, 1960, 82, 2288. 

81. Gambabian, Chebubkov, and Knunyants, Izvest. Akad. Nauk S.S.S.R., 
Otdel. khim. Nauk., 1964, 1626. 

82. Howabd, Sabgeant, and Kbespan, J. Amer. Chem. Soc, 1967, 89, 1422. 

83. Linn, J. Org. Chem., 1964, 29, 3111." 

84. Middleton, Howabd, and Shabkey, J. Org. Chem., 1965, 80, 1375; Howabd 
and Middleton, XJ.S.P. 2,970,173/1961. 

100 



References 

85. Mabtin, J. Chem. Soc, 1964, 2944. 

86. Campaigns:, Chem. Rev., 1946, 89, 1 ; The Chemistry of the Carbonyl Group, ed. 
Patai, Interscienee, New York, 1966, p. 917. 

87. Middleton, J. Org. Chem., 1965, 80, 1395. 

88. Middleton, J. Org. Chem., 1965, 80, 1390. 

89. Middleton and Shabkey, J. Org. Chem., 1965, 80, 1384. 

90. See Chebttrkov and Khtoyamts in Fluorine Chemistry Reviews, ed. Tarrant, 
Arnold (London) and Dekker (New York), 1967, Vol. 1, p. 107 for a review 
of fluoroketene chemistry. 

91. See Bbady and Waters, J. Org. Chem.., 1967, 32, 3703 and references cited 
therein. 

92. Baasch, Chem. Comm., 1966, 577. 

93. Yarovenko, Motornyi, and Kirenskaya, J. Gen. Chem. U.S.S.R., 1957, 27 
2832. 

94. Birchall, Haszeldinb, and Jefferies, unpublished results (see Banks, 
Haszeldinb, and Taylob, J. Chem. Soc, 1965, 5602). 

95. (a) Bbady and Hoff, J. Amer. Chem. Soc., 1968, 90, 6256; (b) Cheburkov, 
Platoshkin, and Knunyants, Doklady Akad. Nauk. S.S.S.R., 1967, 173, 1117. 

96. England and Kbespan, J. Org. Chem., 1968, 83, 816. 

97. Hull, J. Chem. Soc. (C), 1967, 1154. 

98. Chem. Aba., 1967, 66, 115323n. 

99. Selman, TJ.S.P. 3,321,517/1967. 

100. England and Krespan, J. Amer. Chem. Soc, 1966, 88, 5582. 

101. England and Krespan, XJ.S.P. 3,280,150/1966. 

102. Lacey in The Chemistry of Alkenes, ed. Patai, Interscienee, New York, 1964, 
p. 1161. 

103. Cheburkov, Mtjkhamadaliev, and Knunyants, Tetrahedron, 1968, 24, 1341. 



101 



Chapteb 4 

PERFLUOROALKYL DERIVATIVES OF THE 

ELEMENTS 



Perfluoroalkyl derivatives of the following elements have been prepared : 

Group 



I 


II 


III 


IV 


V 


VI 


VII 


VIII 


A B 


A B 


A B 


A B 


A B 


A 


B 


A B 




Li 




B 


C 


N 




O 


F 




Na 


Mg 


Al 


Si 


P 




s 


CI 




K 














Mn 


Fe Co Ni 


Cu 


Zn 




Ge 


As 




Se 


Br 




Kb 










Mo 






Rh Pd 


Ag 






Sn 


Sb 






I 




Cs 


Hg 




Pb 


Bi 






Re 


Ir Pt 



This chapter contains a survey of the synthesis and properties of these 
derivatives 1 except those of the halogens, of carbon, and of the alkali 
metals other than lithium [the last can be visualized as transient inter- 
mediates in reactions associated with perfluorocarbanions generated from 
perfluoro-olefins and the fluorides of sodium, potassium, rubidium, and 
caesium (see p. 33 and the Index)]. 



I. LITHIUM, COPPER AND SILVER 
A. Lithium 

Only a few perfluoroalkyl-lithium compounds have been reported. Tri- 
fluoromethyl-, heptafluoro-n-propyl-, and heptafluoroisopropyl-lithium can 
be prepared by application of the familiar halogen-metal exchange reaction 
to the appropriate perfluoroalkyl iodides : 

-,,,_-. diethyl ether, -78° ■ T . „,. 
R p I+RLi > R p Li+RI 

[R F = CF S , 2 - 8 n-CjF,,* (CF 8 ) a CF;« R = CH a , n-C 4 H„] 

and heptafluoro-n-propyl-lithium can also be obtained directly from the 
iodo-compound and lithium containing 2 % of sodium : 5 

102 



Lithium, Copper, and Silver 

„ „. diethyl ether, - 74° „ _ T . T . T 

u-CJF,I+2Li *■ n-C g F,L I +LiI 

The perfiuoropropyl derivatives can also be generated by metalation of 
the corresponding ftuorocarbon hydrides, e. g., 6 

(CF 3 ) 2 CHF + CH 3 Li a-Wrthr.-TO^ (CFa)20FLi + CH4 

but this method is not preferred to the halogen-metal exchange route, 
which has been employed recently to prepare 1,4-dilithio-octafluorobutane 
from the corresponding di-iodo-compound. 7 Although perfluoroalkyl- 
lithium compounds can thus be prepared in analogous fashion to alkyl- 
lithium compounds, unlike the latter they are not isolable: heptafluoro- 
n-propyl- and heptafluoroisopropyl-lithium decompose to hexafluoro- 
propene (ca. 75%) and lithium fluoride when their ethereal solutions are 
allowed to warm up from — 78° to room temperature, and trifluoromethyl- 
lithium can be decomposed likewise to tetrafluoroethylene and lithium 
fluoride. To account for the formation of tetrafluoroethylene from trifluoro- 
methyl-lithium in this manner, it has been suggested that difluorocarbene 
is formed first then dimerizes: 8 



2:CF 2 > CF 2 :CF 2 

In general, perfluoroalkyl-lithium compounds undergo the addition and 
displacement reactions common to lithium alkyls and Grignard reagents, 
e.g., 

„ „„~ diethyl ether, -45° 
n-C 3 F 7 Li+C 6 H 5 .CHO - > _ 

(n-C s F 7 )(C 6 H 8 )CH.OLi H,S ° 4 ^ 2 °° > (n-C 3 F 7 )(C 8 H 5 )CH.OH(54%) 

(CF 3 ) 2 CFLi + (CH3) s SiCl dlethylether '~ 50 °> (CF 3 ) 2 CF.Si(CH 3 ) 3 (18%) 

but they have not been used widely in synthesis because of the experimental 
difficulties associated with their preparation and manipulation. Syntheses 
involving perfluoroalkyl-lithium compounds are best conducted by adding 
the alkyl-lithium (usually n-butyl-lithium) and the other reactant to an 
ethereal solution of the perfluoroalkyl iodide cooled to — 40° to — 78° ; the 
reaction mixture is then allowed to warm up slowly to room temperature 
and heated under reflux before the product is isolated. 

Perfluorovinyl-litbium, CF 2 :CFLi, is known and its chemistry has been 
reviewed; 9 * it can be prepared 9 b by halogen-metal exchange between per- 
fluorovinyl bromide and an alkyl-lithium or by treatment of trifluoro- 
ethylene with n-butyl-litbium, just as metalation of 3,3,3-trifluoropropyne 

103 



Perfluoroalkyl Derivatives of the Elements 

yields perfluoropropynyl-lithium, CF 3 .C:CLi. 9c Perfluorocycloalkenyl- 
lithium compounds can be generated from l#-polyfluorocycloalkenes and 
methyl-lithium at low temperatures in ether and used in situ to obtain 
stable perfluorocycloalkenyl derivatives,"* e.g., 



-CH 



, ,, diethyl ether f 

[CF 2 L || + CH 3 Li- - *.CH 4 +[CF 2 ], 



-OF 



-CLi 



-CF 



1. CH,.CHO, - 70° 

2. H 3 0+ 



/■ C.CH(CH 8 ).OH 

ICF 2 L ft ( X = 2, 23 % ; x = 3. 42 % ; w = 4, 63 %) 



-CF 



They decompose in the temperature range -10 to 15°, giving lithium 
fluoride and polymeric material; and detailed studies on perfluorocyclo- 
hexenyl-lithium have revealed that this compound probably yields initially 
the transient species perfluorocyclohexyne and perfluorocyclohexa-l,2-diene, 
which can be partly trapped with furan : 10 



2| jjLi -70 to 20° 

2 L 3f furan ' 



-> JUF+ 




fJ 




F 2 



£C 6 F 8 ]„(80%) + 



F 2 F 

? cjI) {u%)+ x ^d (4%> 



Similar evidence has been cited in support of the proposal that perfluoro- 
bicyclo[2,2,l]heptyl-nthium (I), prepared by metalation of l#-undeca- 
fluorobicyclo[2,2,l]heptane with methyl-lithium in ether at —53°, de- 
composes via loss of lithium fluoride and formation of the transient bridge- 
head olefin (II) when allowed to warm up to room temperature. 11 



Li 



F^ 




F 
(I) 




104 






Magnesium, Zinc, and Mercury 
B. Copper 

Perfluoroalkyl-copper compounds, R F Cu, are believed to be formed as 
intermediates in coupling reactions between perfluoroalkyl iodides and 
halogeno-olefins 12 a or aryl halides 12b in the presence of copper bronze, e.g., 

C 7 F 16 I + CHI:CHC1 ° a ' pyri o dine > C 7 F M .CH:CHC1(65%) 

, _ . / \ Cu, dlmethylformamide , , / \ , 

(CF,) 2 CFI + I-f > ^—^ > (CF 8 ) 8 CF-^ )(40%) 



Cu, dim ethyl aulphozide J \^ / V 

** \ / I CF9 a ( ) 



IECFJ.1 + l-( > — j i5 ^ > ( ^CFja-C > (68%) 



Perfluoro-t-butylcopper, (CF 3 ) 3 C.Cu, can be isolated as a solid 2:3 com- 
plex with dioxan from the product obtained by treatment of perfluoro- 
t-butyl bromide with m-(trifluoromethyl)phenylcopper in ether-dioxan 
at 0°, 12c and the isolable acetylide CF 3 .C:C.Cu can be prepared from 
3,3,3-trifluoropropyne and ammoniacal cuprous chloride. 13 * 

C. Silver 

Several perfluoroalkylsilver compounds have been prepared recently by 
nucleophilic addition of silver fluoride across double bonds in perfluoro- 
olefins, and they are much more stable thermally than their alkyl counter- 
parts. For example, treatment of perfluoropropene in acetonitrile with 
silver fluoride at 25° yields perfluoroisopropylsilver, which can be isolated 
as the solvate (CF 3 ) 2 CF.Ag,CH 3 .CN. At 100°, the C— Ag bond in the solvate 
breaks homolytically and the major product (66%) is the fluorocarbon 
(CF 3 ) 2 CF.CF(CF 3 ) 2 , formed by dimerisation of perfluoroisopropyl radicals. 
Treatment of a solution of perfluoroisopropylsilver in acetonitrile with 
hydrogen chloride (or water) or bromine yields the propanes (CF 3 ) 2 CHF 
and (CF 3 ) 2 CFBr, respectively. 13 b 



II. MAGNESIUM, ZINC, AND MERCURY 

A. Magnesium 

No bisperfluoroalkylmagnesium compounds, (R F ) 2 Mg, have been reported, 
but several perfluoroalkyl Grignard reagents, R F .MgX (X = Br, I), have 
been prepared. The latter undergo the types of reaction that are associated 
with their alkyl analogues, but are thermally unstable and must be prepared 
and used at abnormally low temperatures (— 78° to 0°). Because of this 
instability, it is often found convenient to prepare a perfluoroalkyl Grignard 
reagent in the presence of the substrate which it must attack in order to 

8 105 



Perfluoroalkyl Derivatives of the Elements 

effect the conversion required ; in this way the reagent is used up as quickly 
as possible. Preparative and manipulative difficulties appear to have deter- 
red investigators from making much use of perfluoroalkyl Grignard reagents 
in synthesis. 

Perfluoroalkylmagnesium iodides can be prepared at low temperatures 
in donor solvents (e.g., ethers, tertiary amines) from perfluoroalkyl iodides 
and magnesium : 

diethyl ether, - 30° 
R P I+Mg y ' > R,,.MgI(40-80%) 

(R F = CF 3 ," C 2 F 5 ," n-CjF 7 15 - M ) 

while the corresponding bromides are obtained either from perfluoroalkyl 
bromides and magnesium amalgam 17 or from perfluoroalkyl iodides and 
phenylmagnesium bromide : 

diethyl ether, -20° 
CF„Br+Mg/Hg y : ^- CF 8 .MgBr(19%) 

Bpl + C 6 H 5 .MgBr methyl "*""' ~ 78 °> R F .MgBr(40-85%)+C 8 H 6 I 
[R F = C a F 6 » n-C s F„« (CF 8 ) 2 CF«] 

Grignard reagents normally react with compounds containing reactive 
halogen atoms according to the scheme : Z — X + R— MgX' -* Z — R 
+ MgXX', thus the above rapid functional-group exchange reaction be- 
tween a perfluoroalkyl iodide and phenylmagnesium bromide is probably 
due to the presence of 'positive' iodine in perfluoroalkyl iodides ; iodine itself 
reacts with a Grignard reagent in an analogous fashion to a perfluoroalkyl 
iodide: R.MgX + I 2 -* R.I + MgXI. 

The reactions of perfluoroalkyl Grignard reagents proceed normally as 
indicated below; most of the work so far has been carried out with the per- 
fluoropropyl compounds, since these are much easier to prepare than the 
trifluoromethyl or, so it is claimed, pentafluoroethyl compounds. 14 

CO. 

»- R F .C0 2 H 



R p .MgI- 



HCH ° > R p .CH 2 OH 



CH, - P0 -° H '> (CH 3 ) 2 (R F )C.OH 
CF '- C0C1 > R P .CO.CF 8 



C ' F " C0 ' CH '> R F .CO.C 3 F 7 

Little attention has been paid to the conversion of perfluoroalkyl-magnesium 
halides into perfluoroalkyl derivatives of other elements: the literature 
contains an account of the preparation of the pentafluoroethyltin com- 
pounds (C 2 F 5 ) a Sn(CH 3 ) 2 , (C 2 F 5 ) 2 Sn(n-C 4 H 9 ) 2 , and C 2 F 6 .Sn(n-C 4 H 9 ) s from 

106 



Magnesium, Zinc, and Mercury 

pentafluoroethylmagnesium iodide and the appropriate alkyltin chlorides, 19 
and a claim that trifluoromethylmagnesium iodide reacts with silicon 
tetrachloride to yield the dichloride (CF 3 ) 2 SiCl 2 . M Perfluorovinyl bromide 
combines readily with magnesium in tetrahydrofuran at 0° to yield per- 
fluorovinylmagnesium bromide, CF 2 : CF.MgBr, which has been used to 
prepare perfluorovinyl derivatives of a number of elements. 9 

Perfluoroalkyl Grignard reagents are unstable at temperatures above 0° 
and decompose fairly rapidly at room temperature. Decomposition is 
thought to occur via perfluorocarbanion formation, since, for example, both 
l.ff-heptafluoropropane and hexafluoropropene are obtained from hepta- 
fluoro-n-propylmagnesium iodide : 16 

H+ abstraction _ 

,—. : — -* CF 3 .CF,.CHF, 

_,_,,„ + / from solvent 3 * 2 

n-C 3 F 7 .M g I > MgI+CF 3 .CF 2 .CF 2 -— / 

^ — *■ CF 3 .CF:CF 2 

Solutions of trifluoromethylmagnesium iodide decompose to yield fluoro- 
form, tetrafluoroethylene, and polytetrafluoroethylene, possibly according 
to the mechanism : 8 > u 

H+ abstraction „„_ 

/-: ; > CHF a 

__,,,,. + / from solvent 3 

CF 3 .MgI > Mgl + CFg"— <( 

x > :CF 2 ' '> CF,:CF, 



CF 3 -^ g I -£*i^ CF 8 .CF a .CF 2 -M g I -"»*<*», CF 3 .[CF 2 .CF 2 ]^ g I 

An attempt has been made to generate difluorocarbene from trifluoroiodo- 
methane and magnesium amalgam in both the presence and absence of 
diethyl ether and to trap it subsequently as 7,7-difluoronorcarane (III) 
with cyclohexene; 17 only l-iodo-2-trifluoromethylcyclohexane was ob- 
tained, but this result does not preclude the possibility that difluorocarbene 
is formed during the decomposition of a trifluoromethyl Grignard reagent, 
and further experiments are warranted. 




(Ill) 
B. Zinc 



Solutions of the perfluoroalkylzinc halides n-CaFy.ZnBr, 21 n-C 3 F 7 .ZnI, 21 ' 22 
(CF 3 ) 2 CF.ZnI, 4 and n-C 5 F u .ZnI 21 can be prepared by treatment of the 
corresponding perfluoroalkyl halides with zinc dust in peroxide-free dioxan 



107 



Perfluoroalkyl Derivatives of the Elements 

and in 1 ,2-dimethoxyethane at 25-100°; hydrolysis of these solutions with 
water or, more rapidly, with aqueous acid or base affords the corresponding 
monohydrofluoroalkanes, measurement of which enables the yields of the 
zinc compounds to be estimated, e.g., 21 

n-C 3 F,I+Zn di °* an - 1000 > n-C a F 7 .ZnI H ' ' ""*> CF 3 .CF a .CHF 2 (77%) 

Reaction of trifluoroiodomethane with zinc dust in 1 ,2-dimethoxyethane 
yields only fluoroform, 21 and trifluoromethylzinc compounds remain un- 
known. 

Bisperfluoroalkylzinc compounds, (R F ) 2 Zn, are virtually unknown. Per- 
fluoro-1-iodopropane fails to react with zinc in the absence of a solvent at 
temperatures up to 250°; at higher temperatures increasing amounts of 
perfluoro-n-hexane are formed, and no bisperfluoro-n-propylzinc can be 
detected. 22 It has been reported that removal of solvent from a solution 
of perfluoro-n-propylzinc iodide in dioxan gives the solvate n-C 3 F 7 .ZnI, 
C 4 H 8 2 and that this loses dioxan on sublimation in vacuo to yield analyt- 
ically-pure perfluoro-n-propylzinc iodide, which affords only perfluoro- 
propene on pyrolysis: 22 

n-C 3 F 7 .ZnI _i_ 8 °- 200 ° /1 atm - > CF 3 .CF:CF 2 (58%) +ZnIF 

of. 2C 3 H,.ZnI heat/15 °""> (n-Cya,) 1 Zn+ZnI 1 

It has also been claimed 21 that solvates of bisperfluoro-n-propylzinc with 
dioxan and with 1,2-dimethoxyethane can be obtained, together with 
perfhioropropene, by vacuum pyrolysis of the corresponding solvates of 
perfluoro-n-propylzinc iodide, but full details of this work have not been 
published. 

The thermal stability of pernuoroalkylzinc halides is far greater than 
that of the corresponding magnesium compounds; for example, , perfluoro- 
n-propylzinc iodide is stable in dioxan solution at reflux temperatures for 
long periods and can be isolated as a white crystalline solid as described 
above. The chemical reactivity of pernuoroalkylzinc halides lies between 
that of the highly reactive perfluoroalkylmagnesium halides and the 
relatively unreactive perfluoroalkylmercury compounds; and there is a 
greater difference in reactivity between the compounds R.Mgl and R.Znl 
when R = perfluoroalkyl than when R = alkyl. Typical organometallic 
synthetic reactions of pernuoroalkylzinc halides in dioxan solution fail, 
but ketones can be prepared in low yield from the isolated halides or their 
solid dioxinates, e.g., 21 ' 22 

n-C 3 F 7 .ZnI+CH 3 .COCl > n-C 3 F,.CO.CH 3 (18%) 



120° 

n-C 3 F,.ZnI, dioxan +n-C 3 F,.COCl >• (n-C 3 F 7 ) 2 CO(15%) 



108 



Magnesium, Zinc, and Mercury 

Perfluoro-n-propylzinc iodide is unaffected by oxygen, is hydrolysed only 
slowly by water at room temperature to lfl-heptafluoropropane, and reacts 
exothermically but not violently with halogens to yield perfluoro-n-propyl 
halides, e.g., 



n-C 3 F,.ZnI 



CI,, 20-150° 



n-C 3 F,Cl(89%) 



By contrast, lower alkylzinc compounds react vigorously with water at 
room temperature and ignite with halogens and with oxygen. Like alkylzinc 
compounds, perfluoroalkylzinc iodides resist reaction with carbon dioxide. 

In their behaviour towards oxygen and their ability to form stable 1 : 1 
complexes with ethers and amines (e.g., dioxan and pyridine), perfluoro- 
alkylzinc compounds resemble zinc halides rather than alkylzinc com- 
pounds. This illustrates the pseudohalogen character of a perfluoroalkyl 
group and its ability to increase the Lewis acidity of an atom to which it 
is attached. 

Reaction of the olefin CF 3 .CC1:CC1 2 with zinc dust and zinc chloride in 
hot dimethylformamide yields a solution believed to contain the perfluoro- 
propynyl derivative CF 3 .C:C.ZnCl and/or (CFs.CiCJaZn; 28 treatment of the 
solution with cupric chloride provides the diyne CF 3 .C:C.C:C.CF 3 in 46% 
yield. 84 

C. Mercury 

Perfluoroalkyl derivatives of mercury were the first compounds con- 
taining a polyfluoroalkyl group attached to a metal atom to be reported. 
Compounds of the type R F .HgX (R F = perfluoroalkyl; X = halogen, OH, 
N0 3 ) and (R F ) 2 Hg have been prepared, and their physical properties are 
listed in Table 4.1. 

Table 4.1, Physical Properties of Perfluoroalkyl Derivatives of Mercury 



Compound 




m.p. (°C) 


Ref. 


Compound 


m.p. (°C) 


Ref. 


(CF 3 ) 2 Hg 




163 


31 


C 2 F 6 .H g I 


98 


26 


CF 3 .HgCl 




76 


25 


C 2 H 6 .HgOH 


220-225 


26 


CFj.EtgBr 




83 


25 


n-C 3 F 7 .HgI 


770-77-5 


27 


CF 3 .HgI 




112-5 


25 


[<CF 3 ) 2 CF] 2 Hg 


16-2-16-4 


29 


CF 3 .HgOH 


subl 


130/10- 5 mm 


25 


(b.p. 


11 6- 67740 mm) 




CF 3 .HgN0 3 


subl 


100/10- 5 mm 


25 


(CF 3 ) a CF.HgCl 


77-7-78-1 


29 


(Cy^sHg 




106-107 


26 


(CF 3 ) 2 CF.HgOH 


254-265 (dec.) 


29 


C^.HgF 




99-100 


26 


[(CF 3 ) 2 CF.Hg] 2 


292-295 


33 


CjjFj.HgCl 




103-104 


26 


[(CF 3 ) 3 C] 2 Hg 


65-66 


33 


C 2 F 5 .HgBr 




78-79 


26 









1. Perfluoroalkylmercuric Iodides and Related Compounds, R F .HgX. The 

perfluoroalkyl iodides CF 3 I, C 2 F 5 I, and n-C 3 F 7 I react readily with mercury 

109 



Perfluorodlkyl Derivatives of the Elements 

under the influence of u.v. light and/or heat to yield the corresponding 
peifluoroalkylmerciiric iodides, 25-27 e.g., 

„ u.v. light, 160° 
CF a I+Hg — > CF s .HgI(80%) 

C 2 F 6 I+Hg -^U C 2 F B .HgI(88%) 

The reactions may be carried out in sealed Pyrex or silica tubes, and the 
crude products, isolated by ether extraction, are purified by sublimation 
in vacuo. light of wavelength ~ 2200 A decomposes the mercurials; thus 
when silica vessels are used their lower ends must be shielded to prevent 
irradiation of the product which collects there. 

The perfluoroalkylmercuric iodides are white crystalline solids which 
are soluble in common organic solvents such as acetone, alcohol, ether, and 
cyclohexanone, and show a general resemblance to their alkyl analogues. 
Trifluoromethyl- and pentafluoroethyl-mercuric iodide are soluble in water 
(CF 3 .HgI, 61-4 g/1.; cf. CH 3 .HgI, 0-37 g/1. at 25°) and can be recrystallized 
from this solvent; heptafluoro-n-propylmercuric iodide is insoluble in 
water. Aqueous solutions of trifluoromethylmercuric iodide are unstable 
and decompose slowly with the evolution of a gas that is probably mainly 
fluoroform and the deposition of mercurous and mercuric iodides ; no tri- 
fluoromethylmercuric hydroxide, CF 3 .HgOH, is formed so the decomposi- 
tion reaction is not a simple hydrolysis. Fluoroform is evolved slowly when 
aqueous trifluoromethylmercuric iodide is treated with aqueous potassium 
iodide : 

25° 10 davs 

CF s .HgI+3KI+H 2 : *—> CHF 3 (72%)+KOH+K 2 HgI 4 

and no bistrifluoromethylmercury, (CF 3 ) 2 Hg, is formed (cf. alkyl- and aryl- 
mercuric halides: 2R.HgX + 4KI -► R 2 Hg + 2KX + K 2 HgI 4 ). 

like their alkyl analogues, trifluoromethyl- and pentafluoroethyl- 
mercuric iodides are converted into the corresponding basic hydroxides, 
R F .HgOH (R F = CF 3 , C 2 F S ), by the action of moist silver oxide. Aqueous 
solutions of these hydroxides are alkaline to phenolphthalein and react 
with acids to yield salts : 26, 26 ' 28 

R p .HgI ' Ag ° H ' > B F .HgOH HX aq " > B F .HgX 
(R P = CF 3 , C 2 F 5 ; X = F, CI, Br, N0 3 ) 

The nitrates can also be prepared by the action of silver nitrate on aqueous 
solutions of the iodides, e.g., 

CF 3 .HgI+AgK0 3 > CF 3 .HgNO a +AgI 

Heptafluoroisopropylmercuric chloride, (CF 3 ) 2 CF.HgCl, is formed in 54% 
yield when hexafluoropropene is heated with an approximately equimolar 

110 



Magnesium, Zinc, and Mercury 

mixture of mercuric chloride and fluoride in anhydrous hydrogen fluoride 
solution at 82°; it reacts with silver oxide in aqueous ethanol to yield the 
water-soluble basic hydroxide (CF 3 ) 2 CF.HgOH, which reverts to the 
chloride on treatment with concentrated hydrochloric acid but yields an 
unidentified black tar with hydrofluoric acid. 29 Pentafluoroethylmercuric 
fluoride is obtained in 2% yield when tetrafluoroethylene is heated under 
pressure with mercuric fluoride in the absence of a solvent. 30 

Trifluoromethyl- and pentafluoroethyl-mercuric iodides react almost 
quantitatively with iodine at 120° to give the corresponding perfluoroalkyl 
iodide and mercuric iodide, e.g., 

120° 
CF 3 .HgI+I 2 » CF 3 I(92%)+HgI 2 

Similarly, cleavage of the C — Hg bond in heptafluoroisopropylmercuric 
chloride with iodine at 132° gives heptafluoro-2-iodopropane in 30% yield. 
2. Bisperfluoroalkylmercury Compounds, (R F ) 2 Hg. Bisperfluoroalkyl mer- 
curials have been prepared by treatment of perfluoroalkyl iodides and per- 
fluoroalkylmercuric iodides with cadmium amalgam : 31 

C 2 F 5 I Cd/H8 ' 300 > (C 2 F 5 ) 2 Hg(60%) 



CF 3 .HgI -g- d/HB - 12 °- 130 °> <CF 3 ) 2 Hg(90%) 

and by the interaction of terminal perfluoro-olefins with mercuric fluoride 
in the presence of a solvent (see p. 35 for a discussion of the reaction mech- 
anism) : a>. a**. 33 

CF2:CF2+HgFa ^.^^ (OTt ; OTi)iHWM%) 

CF a .CF:CF 2+ HgF 2 85 °> "^"i [(OP Jl OF] i Hg(8 fr -80%, 

(CF 3 , a C:CF 2+ HgF 2 ^ ^cu"' U<"MWW*) 

The standard method for the direct conversion of alkyl halides into dialkyl 
mercurials, namely treatment with sodium amalgam, gives only break- 
down products when applied to trifluoroiodomethane ; application of other 
standard methods to trifluoromethylmercuric iodide, e.g., treatment with 
alkaline ferrous hydroxide, alkaline sodium stannite, aqueous potassium 
iodide (see p. 110), and sodium, also fails to yield the mercurial (CF 3 ) 2 Hg, 
and fluoroform is liberated in reactions with the first three reagents. No 
successful attempt appears to have been made to prepare a bisperfluoroalkyl 
mercurial from a perfluoroalkyl Grignard reagent and mercuric chloride, 
although bisperfluorovinylmercury, (CF 2 :CF) 2 Hg, has been prepared by 
this type of reaction, 34 application of which is the most convenient way to 
prepare dialkyls of mercury. 

Ill 



Perfluoroalkyl Derivatives of the Elements 

Bisperfluoroalkyl mercury compounds decompose on pyrolysis or irradia- 
tion with u.v. light to yield perfluoroalkyl free radicals, e.g., 26 ' 33 

(C 2 F 5 ) 2 Hg U-Y - light > Hg + 2C 2 F 5 > n-C 4 F lc (80%) 

[(CF 3 ) 2 CF] 2 Hg -S=-> Hg + 2(CF S ) 2 CF > (CF 3 ) 2 CF.CF(CF 3 ) 2 (57%) 

They are reduced to monohydrofluoroalkanes by aqueous sodium stannite 
or sulphide, 33 and react with halogens 33 and nitrosyl chloride 36 to give per- 
fluoroalkyl halides and perfluoronitrosoalkanes, respectively, e.g., 

y Ka ' Saq -> (CF 3 ) 2 CHF(60%) 



[(CF 3 ) 2 CF] 2 Hg-^ Ia,1 °°°> (CF 3 ) 2 CFI(74%) 

\noo20^ ( CF 3 ) 2 CF.NO(64%) 
dimethyl- 
formamide 

Reaction of mercurials containing secondary perfluoroalkyl groups with 
sulphur provides a general method for the preparation of perfluorothio- 
ketones (seep. 94). Interestingly, the mercurial [CF 3 .C(N 2 )] 2 Hg, apparently 
prepared from the diazoethane CF 3 .CH.N 2 and mercuric oxide, has recently 
been claimed to yield nitrogen, mercury, and the transient carbyne CF 3 .C : 
when photolysed. 36 

The most valuable property of dialkyl derivatives of mercury is then- 
ability to alkylate other metals [e.g., (n-C 4 H„) 2 Hg + 2Na -* 2n-C 4 H 9 Na 
+ Hg; (CH 3 ) 2 Hg + Mg ->- (CH 3 ) 2 Mg + Hg] ; they also act as mild alkylating 
agents towards reactive halides [e.g., (C 2 H 5 ) 2 Hg + AsCl 3 -*■ C 2 H s .HgCl 
+ CgHg.AsClg]. Disappointingly, bisperfluoroalkyl mercurials have not yet 
proved useful in this way, although there are indications that perfluoroalkyl 
derivatives of magnesium, zinc, and aluminium can be obtained from re- 
actions between bisperfluoroalkyl mercurials and the appropriate metals 
or their amalgams. 29 ' 31 Like a dialkylmercury, though less readily, bistri- 
fluoromethylmercury reacts with mercuric halides to yield the corresponding 
alkylmercuric halides : 

(CF 3 ) 2 Hg+HgX 3 -i2£* 2CF 3 .HgX 
(X = CI, 62% ; X = I, 75%) 

3. Complexes o! Bisperfluoroalkyl Mercurials. Bistrifluoromethylmercury, 
a white crystalline solid, m.p. 163°, is moderately soluble in water (437 g/1.), 
in sharp contrast to dimethylmercury, a water-insoluble covalent liquid, 
b.p. 92°. An aqueous solution of bistrifluoromethylmercury has a small but 
definite conductivity, although it gives no test for mercuric ion and the 
mercurial can be recovered unchanged. The solution is thought to contain 
complex ions such as [Hg(CF 3 ) 4 ] 2 - and [Hg(CF 3 ) 2 OH]-, 31 - 37 a proposal 

112 



Boron and Aluminium 

that gains support from the claim that the mercurial forms complex ions 
of the type [Hg(CF 3 ) 2 X]- and [Hg(CF 3 ) 2 X 2 ] 2 ~ (X = CI, Br, or I) with 
halide ions, as indicated by the results of conductometric titrations and 
the isolation of solid adducts containing large cations. 27 However, cry- 
oscopic and spectroscopic evidence has recently been quoted which in- 
dicates that bistrifluoromethylmercury and halide ions interact in aqueous 
solution to give only weakly associated 'molecular' adducts. 38 

Novel isolable complexes of bisperfluoroalkyl mercurials with both uni- 
and bi-dentate neutral organic donor ligands can be obtained, 39 ' 40 e.g., 

pyridine .y-oxide 

(CFs) * Hg CCl t , 5' " (CF,) 2 Hg(ONC 6 H 5 ) 2 

m.p. 72-75° 

2,2'-blDyridyl 196-200° 

(CF 8 .C0 2 ) 2 Hg ^oJ^og* (CF 3 .C0 2 ) 2 Hg(bipy) ^ > <CF 3 ) a Hg(bipy) 

m.p. 123-124° 

1.10'Dheiurothroline 185° 

(C 3 F 7 .C0 2 ) 2 Hg ■ ] J MW1[ > (C,F 7 .C0 2 ) a Hg(phen) -^ (C 8 F T ) 2 Hg(phen) 

m.p. 170-172° 



III. BORON AND ALUMINIUM 

The literature on perfluoroalkyl derivatives of boron is sparse, and only 
indirect evidence for the successful preparation of aluminium derivatives 
can be quoted. No perfluoroalkyl-thallium compounds appear to be known; 
unsuccessful attempts have been made to obtain them by reaction of per- 
fluoroalkyl iodides with trimethylthallium or with thallous iodide in the 
presence of methyl-lithium. 41 

A. Boron 

1. Tervalent Boron Compounds. Trifluoromethylboron difluoride, decribed 
as an 'enduringly metastable' compound, 42 can be prepared from diborane 
and trifluoromethanesulphenyl chloride (see p. 177) 42 or, preferably, from 
di-n-butyltrifluoromethylboron and boron trifluoride : ** 

i o ti \ nm k — Na alloy , „ -a > x,-^ CF 3 I, -80° 
(n - C4H » )2BC1 (OW, > l"W K (C,Hs),N * 

(n.C 1 H,) 2 B.CF s , (C 2 H 6 ) 3 N l«2!™ <n-C 4 H,) 2 B.CF 3 -2£* 

CF 3 .BF 2 (overall yield ca. 5%) 

Pure trifluoromethylboron difluoride is stable for months if stored in vacuo 
under sterile conditions. 43 In the presence of oxygen, moisture, glyptal 
resin, or other adventitious impurities, however, it decomposes rapidly and 

113 



Perfluoroalkyl Derivatives of the Elements 

quantitatively to boron trifluoride and a polymer thought to be [— CF 2 — ] ; 
this decomposition may occur via internal nucleophilic attack on the boron 
atom by a fluorine atom of the trifluoromethyl group : 

F 2 C— BF 2 >■ BF„+:CF 2 > [— CF 2 — ]„ 

Decomposition of trifluoromethylboron difluoride in the above manner 
conforms with the opinion voiced some years ago that covalent compounds 
containing B— Ry (R F = perfluoroalkyl) bonds will tend to break down 
with formation of substances containing B— -F bonds. 42 -* 4 It was also 
suggested that stable compounds containing B — R F bonds might be ob- 
tained if the acceptor properties of the boron could be eliminated or sub- 
stantially reduced, and on this basis it was argued that trifluoromethyl- 
boron difluoride, CF 3 .BF 2 , should prove to be more stable than tristri- 
fluoromethylboron, (CF 3 ) 3 B, since in the former compound the possibility 
exists for partial donation of lone-pair electrons from the fluorine atoms 
of the BF 2 group into the vacant 2p 2 -orbital of the boron atom. Halogen- 
boron dative p„—p„ bonding has long been thought to occur in boron 
trifluoride : 



f— b; 

-F 



F + - F 

« > F=B<.. -« *■ etc. 



and is invoked to explain the observed weakness of boron trifluoride as a 
Lewis acid compared with boron trichloride and boron tribromide. 45 

A 

B.C 3 F,-n [(CFsJjNJaB.CsFj-n 



(IV) (V) 

In keeping with the above argument, the perfluoro-n-propyl deriva- 
tives (IV) and (V), prepared in moderate yields (~30%) from the corre- 
sponding B— CI compounds and perfluoropropyl-lithium, 46 are fairly stable 
thermally since the susceptibility of the boron atoms towards nucleophilic 
attack by a-fluorines is much reduced through the effect of ^-bonding to 
the adjacent oxygen or nitrogen atoms. Thus the benzodioxaborole (IV), 
for example, can be recovered unchanged after storage at 120° for 3 hours, 
although like (V) it is readily and quantitatively decomposed by aqueous 
alkali to l-ff-heptafluoropropane. 

Apparently 43 di-n-butyltrifluoromethylboron decomposes in similar 
fashion to trifluoromethylboron difluoride, but, like the latter, can be 
stabilized through co-ordination to triethylamine. Trifluoromethylboron 

114 



Boron and Aluminium 



difluoride forms a 1 : 1 complex with dimethyl ether from which it can be 
extracted by trimethylamine ; it cannot be displaced from its etherate by 
boron trifluoride, which thus appears to be the weaker Lewis acid, as would 
be predicted from a consideration of the relative possibilities for fluorine- 
boron p„ — p„ bonding in the two compounds and its effect on the acceptor 
properties of the boron atoms. 

Several simple perfluorovinylboron compounds have been prepared, as 
follows: 47 



CTj.-CT.M&Br 40-50°, tetrahydrofnran 
(CF 2 :CF) a Sn(CH g ) a (65%) 



BCls 
(in excess) 



70° 



70° BC1 * 

(equimolar amount) 



(VI) CP 2 :CF.BCl2(93%) 



(85%)(CF 2 :0F) 2 BC1 



(VIII) 



SOTa 



-23° 



50° 



(CEs:CF) 8 Sn(CH 3 ) 3 



(VII) CF 2 : CF. BF 2 (59 % ) 



(100%)(CF 2 :CF) 3 B 



(IX) 



They are colourless, air-sensitive compounds which are hydrolysed to tri- 
fluoroethylene by hot water but show reasonable thermal stability. Thus 
the gaseous difluoride (VII) (b.p. — 14°) decomposes with the formation 
of boron trifluoride at the rate of 5% per week at room temperature, while 
under similar conditions the trisperfluorovinyl compound (IX) shows no 
sign of decomposition but quantitatively yields the products CF 2 :CF.BF 2 
and BF 3 on being heated to 100° for 5h. Perfluorovinylboron dichloride (VI) 
withstands a temperature of 100° for 5h, but it partly decomposes to boron 
trifluoride when left at room temperature for several days; bisperfluoro- 
vinylboron chloride (VIII) rapidly and completely disproportionates at 
room temperature : < 

2(CF 2 :CF) 2 BC1 — T ' r °° m temp - > (CF 2 =CF) 3 B + CF 2 =CF.BC1 2 

The decomposition of perfluorovinylboron dichloride to boron trifluoride 
and the high stability of vinylboron difluoride, CH 2 :CH.BF 2 , 48 has led to 
the suggestion 47 that the decomposition of perfluorovinylboron difluoride 
to boron trifluoride does not involve disproportionation but a fluorine shift 
from the perfluorovinyl group to the electron-deficient boron atom. The 
tendency for a perfluorovinylboron compound to decompose in this manner 
might be expected to be less than in the case of the corresponding perfluoro- 
alkylboron compound, since the possibility exists for the electrophilic 
character of the boron atom in the former to be reduced by contribution 



115 



Perfluoroalkyl Derivatives of the Elements 

of ^-electron density from a perfluorovinyl group to the vacant p„-orbital 
of the boron. Several perfluorovinylborazines have been synthesized (from 
perfluorovinyl-lithium and the corresponding B— CI compounds) and shown 
to be considerably more stable thermally than the boranes discussed above. 49 

Since perfluoroalkyl- and perfluorovinyl-boron compounds are inter- 
esting from the viewpoint of structure-stability relationships and mode 
of decomposition, it is hoped that efforts to prepare compounds of the 
type (R F ) 3 B and (Rj.) 2 BX will continue to be made, and that more detailed 
studies on trifluoromethylboron difluoride and perfluorovinylboron com- 
pounds will be engaged upon. 

2. Trifluoromethylfluoroborates. Stable compounds containing the tri- 
fluoromethylfluoroborate anion, [CFa.BFg]-, in which the acceptor proper- 
ties of the boron atom are fully satisfied, have been prepared as follows : m 

<CH 3 ) 3 Sn.Sn(CH s ) 3 + CF 3 I "^ " ght > <CH 3 ) 3 Sn.CF 3 (80%) +(CH 3 ) 3 SnI 
(CH 3 ) 3 Sn.CF 3 BF3 ' CC1,80ln -> <CH 3 ) 3 Sn+[CF 3 .BF 8 ]- -^V K+[CF 3 .BF 3 ]- 



cation-exchange 
resin 



NH 4 +[CF 3 .BF 3 ]- 



H+[CF 3 .BF 3 ]-aq. 



Ba 2 +[CF 3 .BF 3 ] 2 - 

Aqueous solutions of trifluoromethylfluoroboric acid are stable, even at 
their boiling points, and strongly alkaline solutions are likewise stable. 
However, the trifluoromethylfluoroborate ion is destroyed in boiling 50% 
sulphuric acid, the fluorine appearing as fluoride ion and not as fluoroform. 
Pyrolysis of potassium trifluoromethylfluoroborate at 300-350°, in vacuo, 
yields potassium fluoroborate and tetrafluoroethylene, together with a 
trace of perfmorocyclopropane ; pyrolysis at 450° yields perfluorocyclo- 
butane as the major volatile product. The mechanism of this decomposition 
is thought to involve nucleophilic attack on boron by a fluorine atom of 
the trifluoromethyl group with the production of difluorocarbene : M 



KlCFa-BFsT -+ K + [BF 4 ]-+:CF 2 ► CF 2 :CF 2 - 



:CF. / \ 
— >- F 2 C CF 2 



CF«— CF, 



CF 2 — CF 2 
116 



Silicon, Germanium, Tin, and Lead 

B. Aluminium 

No perfluoroalkyl derivatives of three-covalent aluminium have been 
described in the literature, although formation of 2H-heptafluoropropane 
when the product formed by interaction of bisperfluoroisopropylmercury 
with aluminium is hydrolysed suggests that a perfluoroisopropyl-aluminium 
compound is produced initially. 29 As in the case of boron, such derivatives 
are likely to be inherently unstable unless the tendency for fluorine to 
migrate from carbon to aluminium can be counteracted. Thus treatment 
of ethereal solutions of lithium aluminium hydride with perfluoroalkyl 
iodides at -78° appears to give complexes of the type LiAl(R F ) 2 I 2 via 
three-step reaction sequences, 81 e.g., 

LiAlH 4 +n-C 3 F,I > LiAl(n-C„F,)H 2 I+H 2 

(X) 
LiAl(n-C 3 F 7 )H 2 I+n-C 3 F J I ► LiAl(n-C 3 F 7 )HI 2 +C 3 F,H 

LiAl(n,C 3 F,)HI 2 +n-C 3 F 7 I > Li Al(n-C 3 F 7 ) 2 I 2 + HI 

(XI) 

The postulated perfluoropropyl-aluminium complex (X), formed from equi- 
molar quantities of lithium aluminium hydride and perfluoropropyl iodide 
in ether at —78°, decomposes violently to aluminium, lithium iodide, 
hydrogen, and lI?-heptafluoropropane if an attempt is made to isolate it 
by evaporation of the ether; addition of water to an ethereal solution of 
the complex yields lfi-heptafluoropropane : 

LiAl(n-C 3 F,)H 2 I + 3H a O — ^U CF 3 .CF a .CHF 2 +LiI+Al(OH) 3 +2H 2 

Reaction of tetrahydrofuran solutions of lithium bis(heptafluoro-n-propyl)- 
di-iodoaluminate (XI), formed by treatment of lithium aluminium hydride 
with a three-molar quantity of perfluoropropyl iodide at —78°, with 
triethyl-lead chloride or triethyltin chloride is believed to produce hepta- 
fluoro-n-propyl derivatives of lead and tin, respectively, but no definite 
products have been isolated and characterised yet. 62 

A trispei^uorovmylaluminium-trimethylamine adduct can be prepared 
as follows : 

3(CF 2 :CF) 2 Hg + 2AlH 3 ,N(CH 3 ) 3 -^1* 2(CF 2 :CF) 3 A1, N(CH 3 ) 3 +3H 2 +3Hg 

It is a colourless, air-sensitive liquid, which decomposes slowly at 20° and 
is hydrolysed by hot water to trifluoroethylene. 83 

IV. SILICON, QEBMANITJM, TIN, AND LEAD 
A. Silicon 

The literature contains only limited and partly indefinite information 
on perfluoroalkyl derivatives of silicon; more is known about polyfluoro- 
alkyl derivatives of silicon, 64 and interest in these compounds has led to 

117 



Perfluoroalkyl Derivatives of the Elements 

the development on a commercial scale of a methyI-3,3,3-trifluoropropyl- 
silicone rubber, [(CF 3 .CH 2 .CH 2 )(CH 3 )Si.O.] B , known as Silastic LS-53, by 
the Dow Corning Corporation. Silastic LS-53 has a working temperature 
range of - 68 ° to 205 ° and is distinctly less soluble in hydrocarbons and other 
solvents than the conventional dimethylsilicone rubbers. 

Some investigators claim, without apparent justification, that com- 
pounds of the type (R F ) iC SiX 4 . a! (R F = CF 3 , C 2 F S , C 3 F 7 ; X = CI, Br, or I; 
x = 1, 2, or 3) can be obtained by passage of the appropriate perfluoroalkyl 
halides over heated copper-silicon alloys, 66 while others report that such 
reactions tend to yield compounds containing only carbon, silicon, and 
fluorine, 20,64 ' 56 e.g., 

CF 3 Br 8i/Cn » 400 - 500 ; CF,SiF 3 (5-8%) 

and that a mixture of fluorine and silicon tetrafluoride will react with 
calcium carbide at 300° to yield a product alleged to contain the com- 
pound C 4 F 9 .SiF 3 . 67 Well-characterized perfluoropropyl derivatives of silicon 
have been described as products of reactions between perfluoropropyl- 
lithium compounds and silicon halides, 3, 4 ' 67 e.g., 

n-C 3 F 7 Li + (C 2 H 6 ) 2 SiCl 2 ether ' ~ 5 °% 

n-CgFj.SiCCijH^^UlsroJ-l-^.C^^SiCCaHJ.CIOyo) 

and the compounds CF 3 .SiCl 3 and (CF 3 ) 2 SiCl 2 are said to be formed when 
trifluoromethyl-lithium or trifluoromethylmagnesium iodide is treated with 
silicon tetrachloride. 20, 5S 

In keeping with the pseudohalogen character of a perfluoroalkyl group, 
dilute aqueous sodium hydroxide liberates the trifluoromethyl groups from 
the compounds CF 3 .SiCl 3 and (CF 3 ) 2 SiCl 2 as fluoroform, 59 and similarly hot 
methanolic base reacts with the compound (n-C 3 F 7 ) 2 Si(C 2 H 5 ) 2 to yield \H- 
heptafluoropropane. Perfluoroalkylsilicon halides are claimed to react with 
water to give perfluoroalkyl-silicones and -polysiloxanes, 55,56 but no detailed 
information is available. Aqueous hydrolysis of polyfluoroalkylsilicon di- or 
tri-halides definitely yields polyfluoroalkyl-silicones or -polysiloxanes, whereas 
aqueous alkaline hydrolysis of all of these types of compound gives almost 
quantitative yields of the appropriate hydrofluoroalkanes, e.g., 69,60 

CHa.SiHGU HaO 
•> CHF 2 .CF 2 .Si(CH 3 )CI 2 (98%) > 



CF 2 :CF 2 - 



u.v. light 

[(CHF 2 . CF 2 ) (CH 3 )S1 . .]« 
silicone (98%) 
SiHCIs H2O 
> CHF 2 .CF 3 .SiCl 3 (44%) > [CHFa.CFa.Si.Oi.s]* 

u.v. light 

polysiloxane (~ 100%) 

^~A /^l / - H a O 

OH- Si— CF 2 — CHF 2 > HO— Si +CF 2 — CHF 2 ;► CHF 2 — CHF 2 



118 



Silicon, Qermanium, Tin, and Lead 

In the case of the compound CHFCl.CF 2 .SiCl 3 , 61 but not of CF 2 C1.CF 2 . 
.SiCl 3 , M cleavage of the C— Si bond with aqueous alkali results in the forma- 
tion of a small amount of an olefin as well as the expected ethane derivative ; 
to explain these facts, it has been suggested that in the hydrolysis of 
CHFCl.CF 2 .SiCl 3 a concerted elimination reaction of the type observed 
with many /S-chloroalkylsilicon compounds : 8a 

r>\ X — M Ot / 

OH- Si— CF 2 — CHF— CI >- HO— Si + CF 2 =CHF+C1- 

A \\ 

r>\ /■ — ^ n* / 

cf- OH- Si— CHa— CH 2 — CI > HO— Si + CH 2 =CH 2 +Cl- 

A l\ 

or chloride-ion elimination from the carbanion CF 2 .CHFC1 must accompany 
the main displacement reaction : 

H a o 
^\ p, . I ► CHF 2 -CHFCI 

OH- Si— CF 2 — CHFC1 • > HO— Si + CF 2 — CHFCI 

A 1\ 



— ci- 

> CF 2 =CHF 



Pyrolysis of polyfluoroalkyl derivatives of silicon containing a-fluorine 
substituents leads to the formation of carbenes. 63 " 65 Thus pyrolysis of the 
compound CFCl 2 .CF 2 .SiCl 3 at 185° yields the olefins CFC1:CFC1 and 
CF 2 :CC1 2 in 80% and 7% yield respectively; the main primary step in 
this pyrolysis is considered to be an internal nucleophilic attack on silicon 
by an a-fluorine atom, followed, or possibly accompanied, by migration of 
a chlorine atom from the /5-carbon atom to g^.ve the olefin CFC1.CFC1: 

185° JT* 

CFC1 2 — CF— SiCls »■ SiFCI 3 +CFCH-CF — > CFC1=CFC1 

F ' CI ' 

The olefin CF 2 :CC1 2 is formed through the occurrence of a small amount 
either of fluorine migration from the /S-carbon atom during the rearrange- 
ment of the carbene CFC1 2 .CF or of the ^-elimination process 

CCJa— CF 2 

^-' > CCI 2 =CF 2 +SiFCl3 



i^^tiCl 3 



The nature of the products of photolysis of the diazoalkane CF 8 .CHN a 
provides evidence for migration of a fluorine atom, involving fission of a 
very strong C— F bond, during a carbene rearrangement reaction: 4 * 



u.v. light .. / 

-Ju^T N 2+ CF S .CH-Y 



F migration „ 
— — ► CF 2 :CHF(32%) 



\ dimerization c is- and trans- 

*" CF 3 .CH:CH.CF 8 (48%) 



119 



Perfluoroalkyl Derivatives of the Elements 

and internal nucleophilic attack on silicon by a fluorine substituent in the 
/9-position has been established, 67 e.g., 

CHF— CH 2 



F^« 



* r 180° 

V > CHF=CH 2 + SiF 4 



SiF 3 



Pyrolysis of 1,1,2,2-tetrafluoroethyltrifluorosilane yields difluoromethyl- 
fluorocarbene, presumably in the singlet state since it reacts stereospecific- 
ally with cis- or <raws-but-2-ene : 65 

u.v. light SbF, 150° 
CF a :CF 2 + SiHCl 3 > CHF 2 .CF 2 .SiCl 3 > CHF 2 .CF 2 .SiF„ » 



no trap 
>• CF 2 :CHF 



S iF 4 + CHF 2 .CF-< fln 

x ► /\7\ 

CF.CHF 2 

In contrast to halogenoalkyl silicon compounds with fluorine in the <z- 
or /J-position, which normally decompose at temperatures below 200° by 
unimolecular transfer of fluorine to silicon as outlined above, compounds 
in which fluorine is present only in the y-position, or further removed from 
the silicon atom, e.g., CF 3 .CH 2 .CH 2 .SiCl 3 , (CF 3 .CH 2 .CH 2 ) 4 Si, decompose 
only slowly, if at all, below 300°. A detailed study of the thermal gas-phase 
decomposition of 3,3,3-trifluoropropyltrifluorosilane in the range 550-640° 
has shown that breakdown occurs by a complex radical-chain mecha- 
nism. 88 

In passing, it is interesting to note that /3-elimination reactions of the 
type discussed above appear to occur during the pyrolysis of fluoroalkyl- 
boron compounds, e.g., 69 



CF 2 — CHCHs' 



V 



k-> 



100° 



> CF2=CHCH 3 -KCH 3 )2BF 

*(CH 3 )2 



and that pyrolysis of the compound CCl 3 .SiCl 3 yields dichlorocarbene, 
which can be trapped if the reaction is carried out in the presence of cyclo- 
hexene: 64 

250° cycfohexenc i iv. _ , ^ 

CCla-SiCIs > SiCl 4 + :CCI 2 > P;CCl2(60%) 



B. Germanium 

Alkyltri-iodogermanes, e.g., CH 3 .GeI 3 , C 2 H s .GeI 3 , are best prepared by 
the action of alkyl iodides on germanous iodide at elevated temperatures 

120 



Silicon, Germanium, Tin, and Lead 

in sealed tubes. Use of trifluoroiodomethane in this reaction yields tri- 
fluoromethyltri-iodogermane together with a trace of bistrifluoromethyldi- 
iodogermane, 70 and there seems no reason why other perfluoroalkyliodo- 
germanes should not be capable of preparation in similar fashion: 

CF 3 I + GeI 2 ► CF,.GeI,(54%)+(CF s ) i! GeI a (0-5%) 

Treatment of trifluoromethyltri-iodogermane, a dense, yellow oil, m.p. 
8-4°, b.p. 40-42 °/10 -3 mm, with silver chloride or fluoride converts it into 
the corresponding trihalogenogermane : 

CF 3 .GteI, AgX ' 8 ~ 2 °> CF 3 .GeX 3 
(X = F or CI) 

The C— Ge bonds in the compounds CF 3 .GeF 3 , CF 3 .GeCl 3 , and CF 3 .GeI 3 
can be cleaved with aqueous alkali at room temperature with quantitative 
formation of fluoroform, e.g., 

NaOH aq., 1-2 days 
— =% CHF 3 (100%) 



and hot water effects the same hydrolysis. With cold water, the trichloro- 
and trifluorp-compound immediately give clear solutions which are stable 
at room temperature for at least two days ; the tri-iodo-compound, although 
not immediately miscible with cold water, gives a clear solution on pro- 
longed shaking, and this slowly evolves fluoroform. The nature of these 
solutions is not established, but addition of potassium fluoride solution to 
an aqueous solution of either trifluoromethyltrifluorogermane or trifluoro- 
methyltri-iodogermane results in immediate precipitation of potassium 
trifluoromethylpentafluorogermanate, K 2 CF 3 GeF 5 . 

Trifluoromethyltri-iodogermane decomposes slowly at 180° to yield a 
mixture of tetrafluoroethylene, perfluorocyclopropane, perfluorocyclo- 
butane, and germanium tetra-fluoride and -iodide. The nature of these 
products suggests that this pyrolysis causes a-elimination of fluorine to 
occur with the formation of difluorocarbene (c/. trifluoromethylboron 
compounds, pp. 114, and polvfluoroalkylsilicon compounds, p. 119): 



CF 2 — Gel 3 - 

! t 


180 


-> :CF 2 + GeFI 3 


4GeFI 3 - 




->- GeF 4 +3GeI 4 








F 2 
cf* y°\ 

•, T\,p pp. 








2:CF 2 • 




> rPn-rKi 








CF a :CF, CF 2— CF 2 






> 1 1 
CF 2 — CF 2 



121 



Perfluoroalkyl Derivatives of the Elements 

C.Tin 

Mono- and bis-perfluoroalkyl derivatives of tin are known (see Table 4.2). 
Examples of both types have been prepared from pentafluoroethylmag- 
nesium iodide, e.g., 

2C 2 F 5 .MgI + {OH.J.SnCB, ^^^f^ (C 2 F B ) 2 Sn(CH 8 ) 2 (34 % ) + 2MgClI 

tetrahydrofuran 
C 2 F 5 .MgI + (n-C 4 Hy,8na J^. ► C 2 F 5 .Sn(n-C 4 H 9 ) 3 (48%)+MgClI 

and the former type can be obtained by the action of perfluoroalkyl iodides 
on hexa-organoditin compounds tinder the influence of heat or, preferably, 
u.v. radiation, e.g., 

CF 3 I + (CH 8 ) 3 Sn.Sn(CH 3 ) 8 "^ " ght > CF„.Sn(CH 3 ) 3 (80%)+(CH 8 ) 8 SnI 

220° 
C 2 F 5 I + (C 6 H 5 ) 8 Sn.Sn<C 6 H 6 ) 8 > C 2 F 5 .Sn(C 6 H s ) 8 (22%) + (C 8 H 6 ) 3 SnI 

Tetrafluoroethylene also reacts with hexamethylditin in the presence of 
u.v. light to yield an oil which analyses correctly for the compound 
(CH 3 ) 3 Sn.CF 2 .CF 2 .Sn(CH3)3, while a similar reaction involving the colour- 
less modification of diphenyltin, [(C 6 H 5 ) 2 Sn] 5or6 , gives a solid of empirical 
formula [(C 6 H 5 ) 2 Sn.CF g .CF 2 ] but unknown molecular weight. 74 

Table 4.2. Physical Properties of Perfluoroalkyl Derivatives of Tin 



Compound 


m.p. (°C) 


b.p. CC) 


Ref. 


CF 3 .Sn(CH 3 ) 8 


— 50-9 


100-0* 


50, 71, 72 


CF 3 .Sn(CH 3 ) 2 Cl 


46-47 


— 


71, 73 


CgF 5 .Sn(CH,) 8 


— 


107-0* 


72 


C 2 F 6 .Sn(C 2 H 6 ) 3 


— 


177-4* 


72 


C 2 F 5 .Sn(n-C 4 H 9 ) 3 


— 


48/0-035 mm 


19 


C 2 F 6 .Sn(C e H 6 ) 3 


127-128 


— 


72 


C 2 F 6 ) 2 Sn(CH 3 ) 2 


— 


62-63/89 mm 


19 


(C.Fg^SnCn.^H,), 


— 


42/1 mm 


19 


[(C 6 H 5 ) 2 Sn.OF 2 .CF 2 ] a: 


128 


— 


74 



Perfluoroalkyl derivatives of tin are thermally unstable, but so far the 
pyrolytic decomposition of only trrmethyltrifluoromethyltin appears to 
have been studied in detail. Although it can be distilled without decom- 
position, this derivative is converted almost quantitatively into a mixture 
of perfluorocyclopropane and trimethyltin fluoride when heated at 150° 
in a sealed tube in the absence of air. 71 The primary step in this pyrolytic 
breakdown appears to be internal nucleophilic attack by an a -fluorine atom 
on the electropositive tin atom, leading to the generation of difluorocarbene 
(c/. the thermal decomposition of polyfluoroalkyl derivatives of silicon and 
of trifluoromethyl derivatives of boron and germanium) ; thus pyrolysis of 

* Calculated from vapour pressure measurements. 

122 



Silicon, Germanium, Tin, and Lead 

trimethyltrifluoromethyltin in the presence of an excess of tetrafluoro- 
ethylene gives perfluorocyclopropane almost quantitatively according to 
the scheme : n 

CF s .Su(CH s ) 8 -^»- (CH 3 ) 3 SnF + :CF 2 -^> F S 0— CF 2 



CF. 



and cyclopropenyl derivatives of the type R a M.C=C(CF 3 ).CF 2 [RjM 
= (CHgJgAs, (CH s ) 8 Si, or (CgH^gGe] can be prepared by heating the 
corresponding trifluoropropynyl compounds, R33i.CiC.CF3, with trimethyl- 
trifluoromethyltin at 140°. 7S 

As expected from the pseudohalogen character of a perfluoroalkyl group, 
alkyl- or aryl-perfluoroalkyltin compounds are readily cleaved by aqueous 
sodium hydroxide with the quantitative formation of the corresponding 
monohydrofluoroalkanes,' e.g., 

HCT (CH 3 ) 3 Sn-^CF s — >- (CH 3 ) 3 Sn.OH+CFi; H '°> CHF 3 
cf- HO _ +(CH 3 ) 3 SnCl — v (CH 3 ) 3 Sn.OH+Cr 

This type of reaction has been extended to provide a convenient route to 
<7em.-difluorocyclopropanes: trimethyltrifluoromethyltin is heated with 
sodium iodide in an aprotic solvent in the presence of an olefin which traps 
the difluorocarbene, formed, presumably, by loss of fluoride ion from tri- 
fluoromethyl anion, 76 e.g., 

NaI + (CH 3 ) 3 Sn.CF 3 n jt "*" xv — >- (CHJ-Snl+Na+CFT ~ NaF > 
a/a s 1,2-dlmethoxyethane x 3/3 8 



tro««-CiH 5 .CH:CH.C8H 7 -n 2 5 \ / H 

/ > H / \y / \3,H,-n 



(74%) 



'At* / (CH a ) 2 C:C(CH,)2 

c **-<r ■ > (CH 3 ) 2 \ 7(CH 3 ) 2 (77%) 



\ C,F 5 .CH:CH 8 



F 



2 



F F 

->■ f/ \ <, (61%) 

F F F 2 

In contrast to nucleophilic reagents, the electrophilie reagents trifluoro- 
acetic acid and chlorine cleave the tin-alkyl or -aryl bonds in appropriate 
perfluoroalkylated compounds; thus pentafluoroethyltriphenyltin releases 
its phenyl groups quantitatively as benzene on treatment with trifluoro- 
acetic acid at 60°, while trimethyltrifluoromethyltin reacts with chlorine 
at —46 to 20° to yield dimethyltrifluoromethyltin chloride and methyl 

9 * 123 



Perfluoroalkyl Derivatives of the Elements 

chloride. However, boron trifluoride cleaves the CF 3 — Sn bond in trimethyl- 
trifluoromethyltin and the salt (CH 3 ) 3 Sn+[CF 3 .BF 3 ]- is formed (see p. 116). 

D. Lead 

Thermal or photochemical reaction of tetra-alkyllead compounds with 
perfluoroalkyl iodides is claimed to yield compounds of the type R F .PbR 3 
(Rj. = perfluoroalkyl, R = alkyl), but details of the preparation of only 
trimethylpentafluoroethyllead are available : 72 

C 2 F 5 I + (CH 3 ) 4 Pb -i^> C 4 F e .Pb(CH 1 ),(28%)+CH,I 

Compounds containing n-C 3 F, — Pb bonds are believed to be formed when 
a tetrahydrofuran solution of lithium bis(heptafluoro-n-propyl)di-iodo- 
aluminate is refluxed in the presence of triethyllead chloride. 62 

Like its tin analogue, trimethylpentafluoroethyllead evolves pentafluoro- 
ethane quantitatively on treatment with aqueous potassium hydroxide, 
and reacts with dry hydrogen chloride to yield methane. 



V. NITROGEN, PHOSPHORUS, ARSENIC, ANTIMONY, 
AND BISMUTH 
A. Nitrogen 

1. Perfluoroalkylamines and Perfluoroalkyl-iV-nuoroamines. Perfluoro- 
alkylamines, i.e. compounds of the type R F .NH 2 , (R F ) 2 NH, and (Rj) 3 N 
(R F = perfluoroalkyl), can be considered as derivatives of ammonia, formed 
by stepwise replacement of hydrogen by perfluoroalkyl groups; from the 
point of view of their chemistry, however, perfluoro tertiary alkylamines, 
(R F ) 3 N, are best classed with perfluoroalkyl -.W-fluoroamines R F .NF 2 and 
(Rj.JgNT', which should be thought of as derivatives of nitrogen trifluoride. 

a. Primary and Secondary Perfluoroalkylamines, Rj..NH 2 and (R F )2NH. 
Compounds containing the >CF.NH a group lose hydrogen fluoride so 
readily that primary perfluoroalkylamines have so far defied all attempts 
to isolate them. For example, 77 * the imine (CF 3 ) 2 C:NH reacts exothermically 
with anhydrous hydrogen fluoride at 0° in polythene to yield a product 
containing (CF 3 ) 2 CF.NH 2 (detected by 19 F n.m.r. spectroscopy), but no 
successful attempt to isolate the amine, which reacts rapidly with glass, 
has been reported; also, treatment of the triazene CF 3 .N: N.NH.C 6 H 6 with 
phenol yields the imine CF 2 :NH, presumably via loss of hydrogen fluoride 
from the primary product, CF 3 .NH g . 77b All primary amines containing a 
CF a .NH 2 group decompose similarly in the presence of base or solvent of 
high dielectric constant and, for example, attempts to prepare them from 
perfluoro-oleflns and ammonia result in the formation of nitriles, e.g. (see 
p. 29 also), 



CF 3 .CF:CF 2 %■ [CF 3 .CHF.CF 2 .NH 2 ] > CF 3 .CHF.CN 



124 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Interestingly, reaction of thiazyl trifluoride, F 8 S:N, with anhydrous hy- 
drogen fluoride yields pentafluorosulphanylamine, SF 6 .NH 8 , which is 
stable when stored at — 78° in dry glass but dissociates slowly to its pre- 
cursors at room temperature and is rapidly destroyed by water. 78 

Perfluorinated secondary amines can be prepared by the reaction of 
perfluoroimines with anhydrous hydrogen fluoride and by the reduction 
of perfluoro-i^-fluoroamines with manganese pentacarbonyl hydride or, 
more simply, hydrogen iodide. The former method has been used to prepare 
both acyclic and cyclic amines, e.g., 79-81 

HI* 25° 

CFj.NrCFu '- >- (CF 3 ) 2 NH(~100%) 

(see p. 134) 

CF 8 .CF CF HT.80' CF 3 .CF CF 2 (87%) 



(see p. 127) 



H 



F 2 F 2 

,C^ __ _. ,C 

HF, 40" 



F 2 C/ ^CF 2 ww ino F 2 C^ \CF 2 



(63%) 



F2C\ N ^CF F 2 C Xn /CF 2 

(see p. 132) H 

but so far the latter has only been employed to synthesise cyclic com- 
pounds, 82 e.g., 

F 2 C/°\CF S ^^.^^ F 2 C/°^CF 2 (67%) 

■f, JL i™ of Hi/molecular sieve, 20° I I 

2 V N X 2 F 2C\ N /CF 2 (45%) 

I I 

F H 

The reactivity of the C:N bond in a perfluoroimine towards hydrogen 
fluoride contrasts markedly with the inertness of the C:C bond in a per- 
fluoro-olefin towards this reagent (see p. 35). 

Bistrifluoromethylamine, (CF 8 ) 2 NH, is a colourless gas which boils at 
— 6°, i.e. 13° lower than dimethylamine. This difference in volatility is 
due in part to the decreased basicity of nitrogen when it is attached to a 
perfluoroalkyl group, an effect which is also responsible for the lack of 
reaction between the amine and hydrogen chloride, trifluoroacetyl chloride, 
trifluoroacetic anhydride, or boron trifluoride. 79b However, boron trichloride 
and tribromide, which are stronger acceptors than the trifluoride (see 
p. 114), react readily with bistrifluoromethylamine to yield the correspond- 
ing bistrifluoromethylaminoboron dihalides, (CF 8 ),N.BX 8 (X = CI or Br), 

125 



Perfluoroalkyl Derivatives of the Elements 

probably via the adduots (CFjOaNH.BXg. 83 Nitration of bistrifluoromethyl- 
amine with a mixture of concentrated nitric acid and trifluoroacetic an- 
hydride provides the nitramine (CF 3 ) a N.N 8 ; 79b perfluoropiperidine and 

perfluoromorpholine (CF a .CFg.X.CF 2 .CF 2 .NH; X = CF 2 or O) undergo 
nitration in similar fashion. 82 

Bistrifluoromethylamine is rapidly hydrolysed by cold water to carbon 
dioxide, with the liberation of fluoride and ammonium ions; similarly, 
perfluoropiperidine and perfluoromorpholine react with water, and more 
readily with aqueous base, to give perfluoroglutaric acid and perfluoro- 
/J-oxaglutaric acid, respectively. The initial reaction in each case is probably 
loss of hydrogen fluoride leading to formation of the corresponding imine, 
which is known to hydrolyse readily and yield the observed products : 

(CF 3 ) 2 NH -=2^ C F 3 .N:CF a -5** CF 3 .NH.CF 2 OH ~ 2HF > CF 3 .NCO H '°> 
(XII) 

CF 3 .NH.C0 2 H -I52V CF 3 .NH a -=!5i FCN -5^ CO a + NH 4 F 




NH 3 + 

HO a C CO a H 



F 2 F 2 F 2 

F»f|F s ~HF F 2 f ^lF, H s Far^^^iFa 

I I " *"\l J < 



H H 



126 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Treatment of perfluoro(methylenemethylamine) (XII) with a limited 
amount of water allows trifmoromethyl isocyanate to be isolated in up 
to 83% yield. 84 As expected from the above, hydrolysis of perfluoro-(2- 
methyl-21?-azirine) (XIII) gives trifluoropyruvic acid [in the form of its 
hydrate, CF 3 .C(OH) 2 .C0 2 H] : M - 8S 

dimethlformamide — _ „ — F - ^ 
CF 3 .CF:CF 2 +N3 ^— — *■ CF 3 .CF.CF 2 -N 3 *■ 



20° H.O 
CF 3 .CF=CF > CFj.CF CF >- CF s .CO.C0 2 H 



A} 



N 



(XIII) 



b. PerfLuoroalhyl Derivatives of Nitrogen Trifluoride — R F .NF 2 , (Rj-JjNF, 
and (R F ) 3 N. (i) Peep abation. Many perfluoroalkyl nitrogen compounds be- 
longing, or related, to this class can be prepared by direct or indirect fluori- 
nation of the corresponding amines, of amine derivatives, or of cyanides. 

Moissan was the first investigator to treat a cyanide with fluorine, but 
he did not characterize the reaction products. 86 More than forty years 
later, in 1936, 87 Ruff and Giese described how they had demonstrated the 
formation of the compound CF 3 .NF 2 in the reaction that occurs between 
fluorine and silver cyanide at 0°; this reaction also gave a product of 
molecular formula C 2 F 6 N 2 , probably hexafluoroazomethane, CF 3 .N:N.CF 3 , 
and trifluoronitrosomethane, CF 3 .NO, which presumably derived its 
oxygen from silver nitrate present as an impurity in the silver cyanide. 
Shortly afterwards, in 1940, 88 Ruff and Willenberg reported that hexa- 
fluoroazomethane can be synthesized by the fluorination of cyanogen iodide 
with iodine pentafluoride, a reaction still recognized as a convenient labora- 
tory method of preparation of the azo-compound. 89 ' 90 Some examples of 
recent work on the fluorination of cyanides and of organic nitrogen com- 
pounds with fluorine or metallic fluorides (see pp. 7, 10) are listed inTable4.3. 

Except in the few cases noted, no real estimate of the yields of the 
products shown in Table 4.3 can be made, but in general they are poor 
(1-15 % ). Poor yields result from the highly energetic nature of the fluorina- 
tion reactions, and from the difficulties which are encountered in the separa- 
tion of the desired products from complex mixtures, which usually contain 
fluorocarbons. Fluorination of some organic nitrogen compounds leads also 
to the formation of highly reactive substances (e.g., CF 2 :NF) which attack 
hydrocarbon stopcock grease and mercury. This causes extra difficulties, 
and such compounds are often destroyed to facilitate purification of more 
stable products, so that a complete picture of the reaction products is not 
obtained. 

127 



Perfhtoroalkyl Derivatives of the Elements 

Table 4.8. Details of the Fluorination of Some Inorganic Cyanides and of Some 
Organic Compounds Containing Nitrogen with Fluorine or Metallic Fluorides 







Perfluorinated 








Fluorinating agent 


nitrogen compounds 


Yield 




Starting material 


(diluent/temp. °C) 


obtained 


(%) 


Ref. 


(ON), 

r 


F 8 (N 2 /200) 


CF S .NF 2 

(CF 3 ) 2 NF 

C2F B .NF 2 

NF 2 .CF 2 .CF 2 .NF 2 

CF 3 .N:N.CF g 

CF 8 .N:N.C 2 F 5 




91 




AgF 2 (-/105-115) 


CF 2 — CF 2 

1 1 

N=N 


90 


92 


HCN 


F 2 (N 2 /150) 


CFg.NF 2 

(CF 8 ) 2 NF 

(CF 8 ),N 

C 2 F 6 .NF 2 

NF2-CF2.CF2.NFg 

CF 8 .N:N.CF S 


60 


91 




CoF 3 (N2/200-250) 


CF S .NF 2 
C 2 F,N 2 




94 


C1CN 


F 2 (N 2 /94) 


CF 2 C1.NF 2 
CF 8 .NC1F 
CF 3 .N:N.CFs 
CF 3 .N:N.CF 2 C1 




93 




AgF 2 (Cl2/- 196 


CF 3 .NC1 2 


30 


93 




to 25) 


CF 8 .N:N.CF 3 
CF S .N:N.CF 2 C1 






CH 3 .CN 


F 2 (N 2 /85-275) 


C 2 F 6 .NF 2 
CF 2 :NF 




95 


CF 2 — CN 


F 2 (N 2 /145) 


t? n nv 


20 


96 


Jc 2*-* ^*-E 2 


1 
CF 2 — CN 




1 
F 2 C\ /CF 2 

N' 

F 
CF S .NF 2 






CH 3 .NH 2 


F 2 (N 2 /100) 




97 






(CF S ) 2 NF 










C 2 F 6 .NF 2 






(CH 3 ) 2 NH 


F 2 (NJ275) 


CF3.NF2 

(CF 8 ) 2 NF 

(CF3) 8 N 

C 2 F 5 .NF 2 

(CF3) 2 N.N(CF 3 ) 2 




97 


(CH 3 ) 3 N 


F 2 (N 2 /275) 


CF 3 .NF 2 

(CF 8 ) 2 NF 

(CFg),N 

C 2 F S .NF 2 

CF3.NF.NF.CF3 

(CF 3 ) 2 N.N(CF 8 ) 2 




97 



128 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 









Perfluorinated 








Fluorinating a 


jent 


nitrogen compounds 


Yield 




Starting material 


(diluent/temp. 


°C) 


obtained 


(%) 


Bef. 




CoF 3 (N a /130-250) 


CF 3 .NF a 




98, 99 








(CF 8 ) a NF 

(CF S ) 8 N 

(OTJbKyjyN 






(CHj.yCjjKON 


CoF 3 (Nj,/250) 






99 


(C.HjJ.N 


CoF a (N a /300) 




(C 8 F 7 ) 8 N 




99 


HCON(CH3) 2 


F a (He/275) 




(CF,) a NF 

(CF 8 ),N 

(CF 3 ) a N.N(CF 8 ) a 


38 


100, 101 


CA.NH,, 


F a (Ng/290) 




C.F u .NF a 




102 




CoFj(N 4 /300) 




C,F M .NF a 


0-2 


102 


U^N 


F a (N a /280) 




F a 


0-3 


102 




CoF 3 (N a /350) 




F 2 (X x CF a 


0-2 


102 


V 






1 1 

F 2 C\ /CF a 

N N 

1 
F 






HjjN.qrNH). 


F a (N 2 , NaF/0) 




CF a (NF a ) a 


10 


103 


.SOjH 












HjN.C^NH). 


F a (N a> NaF/O) 




CF a (NF a ) a 




104 


.NH.CO.NH2 






CF(NF a ) 8 

(F a N) a C:NF 

F a N.CF:NF 


5 
5 





Many perfluoro tertiary amines, some N-&aoro secondary perfluorocyclo- 
alkylamines, and tridecafluorocyclohexylamine, C e F u .N]? 8 , have been pre- 
pared by electrochemical fluorination (see p. 11) of the corresponding 
hydrocarbon amines (see Table 4.4). These products are always accompanied 
by fluorocarbons, formed by degradation of the starting materials, and 
often prove difficult to purify. 

Recently it has been discovered that perfluoroalkyl-J^^-6Muoroamines 
can be prepared from tetrafluorohydrazine, which contains a weak N— N 
bond p^NI^— NE 2 ) « 20 kcal/mole] and yields detectable amounts of 
difluoroamino radicals even at room temperature, 109 and sources of per- 
fluoroalkyl radicals, e.g., 89,110 



O^.N.-N.C^ + N^F, 



450° 



-> 0^^(64%) 



flow pyrolysia 

O 

CF 3 .NO + NjF, + 12 °°> CF 3 .NF 2 (20%) + CF 3 .N:NF(45%) 

(XIV) 
O 

CFgH- N^, °' T ' """> CF 3 .NF 2 (23%), CF 3 .N:NF(6.5%) , CF 3 .NO, I. 



129 



Perfluoroalkyl Derivatives of the Elements 



Table 4.4. Products of the Electrochemical Fluorination of Some Hydrocarbon 
Amines and Their Physical Constants 





Product 










Starting 
material 


(% yield 
if known) 


b.p. 
(°C/mm Hg) 


n' D (*°C) 


e? 4 (*°C) 


Ref. 


(CH3) 3 N 

(C a H 6 ) 3 N 

(C 4 H„)3N 

(CH 3 )(C 2 H 5 ) 2 N 

C 6 H B .NH 2 


<CF 3 ) 3 N 
(C 2 F 5 ) 3 N(27) 
(C 4 F„) 3 N* . 
(CF 3 )(C 2 F 6 ) 2 N 
<VP n .OT, 


— 11/735 

68/743 

177-2/756 

45/734 

77 


1-258(25) 
1-290(25) 
1-253(25) 
1-292(20) 


1-708(35) 
1-856(25) 


105 
105 
105 
105 
106 


iT^i 


F 2 
F 2 C X X CF 2 












1 1 O 


49-5 


1-2752(20) 


1-7043(25) 


107 



^ 



F 2 C 






OF, 



F 2 C X X CF 2 



F 2 C 



| (44) 65-0-65-5/730 1-275(24) 1-760(24) 108 



\ N /' 



CF 2 




F 2 C 



CF 3 



CF 2 



(19) 15/356 



F * C \ N / CF z 



107 



In the last reaction, the nitric oxide necessary for the formation of JT-fliioro- 
iV'-trifluoromethyldiazine-^'-oxide (XIV) and trifluoronitrosomethane 
arises from attack of tetrafluorohydrazine on the silica reaction vessel em- 
ployed. (J\r,2V-Dmuoroalkylamines, e.g., CH 8 .]SnF 2 , C 2 H 5 .NF 2 , (CH 3 ) 3 C.NF 2 , 
can be prepared by the interaction of tetrafluorohydrazine with alkyl- 
radical sources. 111 ) However, the best method of preparation of per- 
fluoroa!kyl-^,^-difluoroamines seems to be direct fluorination of fluoro- 
carbon nitriles or imines at low temperatures in the presence of caesium 
fluoride, 149 e.g., 



CF..C-N+2F, 



CsF, -78" 



monel cylinder 



> CF3.CF a .NF a (~100%) 



PsI? — 78° to 25° 

(CF 3 ) 2 C:NH + 2F 2 : > (CF 8 ),CF.NF 2 (97%) 

v 3 ' 2 2 monel cylinder 8 ' 2 av 

* This perfluoro tertiary amine is marketed by the Minnesota Mining and Manu- 
facturing Co. under the name Fluoroehemioal FC-43. 



130 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Reaction of chlorine monofluoride with fluorocarbon nitriles yields per- 
fluoroalkyl- JVV^-dicMoramines, 161 e. g., 

C 2 F 5 .C:N+C1F ~ 78 ° „ > C a F s .CF 2 .NCl a (65%) 

monel cylinder 

(ii) peopeeties ast> beactions. Perfluoroalkyl derivatives of nitrogen 
trifluoride are colourless, almost odourless, non-basic, water-insoluble 
compounds which resemble saturated fluorocarbons in many respects. 
They are dense, highly volatile, have low refractive indices and surface 
tensions (see Table 4.4), and — this applies mainly to perfluoro tertiary 
amines — resist chemical attack. Interestingly, replacement of a >CF — group 
in a saturated fluorocarbon by a nitrogen atom scarcely affects the boiling 
point, as illustrated by the following data; 

CF 4 , b.p. -128°; NF 3 , b.p. -129° 

C 2 F 6 , b.p. -78°; CF 3 .NF 2 , b.p. -75° 

C 3 F 8 , b.p. -38°; C 2 F B .NF 2 , b.p. -38°; (CF 3 ) 2 NF, b.p. -37° 

This also applies to replacement of a =CF — group in a terminal perfluoro- 
olefin, but not in an internal olefin, by the structure =N— , e.g., cf. 
CF 3 .CF:CF 2) b.p. -29° and CF 3 .N:CF 2 , b.p. -33-7°; C a F 5 .CF:CF 2 , b.p. 
1 ° and C 2 F 5 . N:CF 2 ,b.p.-6 ° ; CF 3 .CF : CF.C F 3 ,b.p.0°and CF 3 .N : CF.CF 3 , 

b.p. - 15°; CF 2 .[CF 2 ] 3 .CF:CF, b.p. 53° and CF~{GF^N^CF, b.p. 40-7°. 

Perfluoro tertiary amines appear to be devoid of basic properties : they 
form neither salts with acids nor coordination compounds with boron tri- 
fluoride. This is to be expected in view of the severe restriction on the 
availability of the unshared electron pair on a nitrogen atom attached to 
three powerfully electron-attracting perfluoroalkyl groups. 

No reaction occurs between a perfluoro tertiary amine and hot con- 
centrated acids, alkalis, or oxidizing agents, but it can be decomposed by 
fusion with sodium or potassium to a mixture of carbon and alkali-metal 
cyanide and fluoride. This decomposition can be effected quantitatively 
in a sealed nickel bomb at 600-650°, and is used to determine the nitrogen 
and fluorine content of perfluoro-amines. 118 The nitrogen content alone is 
best determined on a routine basis by a modified Dumas procedure : U3 in 
essence, a weighed sample of the perfluoro-amine is vaporized into a slow 
stream of carbon dioxide which carries it through a silica tube packed 
with either a cupric oxide-copper mixture at 850° or nickel oxide at 1050°, 
and the nitrogen liberated is collected and measured in a nitrometer filled 
with 50% aqueous potassium hydroxide solution. A carbon determination 
on a perfluoro tertiary amine can be carried out by the method used for 
fluorocarbons (see p. 17). All these analytical methods are applicable to 
any organic compound containing nitrogen and fluorine. 

131 



Perfluoroalkyl Derivatives of the Elements 

Controlled pyrolysis of perfluoro tertiary amines in carbon 114 or plati- 
num 118 tubes gives rise to perfluoro(alkylenealkylamines), e.g., 

(n-C 4 F 9 ) s N ptt ° be >- CF 3 .[CF 2 ] 2 .CF:N.CF 3 (43%) + fluorocarbons 

Compounds of the latter class can also be prepared by reductive defluorina- 
tion of appropriate 2V-fluoroamines with ferrocene, a reagent which effects 
partial defluorination of perfluoroalkyl-^^-difluoroamines to the corre- 
sponding i^-fluoroimines, 11 * e.g., 

C 2 F e .NF.CF 2 .CF, + 2(C 5 H 6 ) 11 Fe — 51> C 2 F 5 .N:CF.CF 3 (84%) + 2(C 5 H 6 ) 2 Fe*F- 

26° 

n-C 3 F 7 .CF 2 .NF 2 +2(C 5 H s ) 2 Fe -^-> n-C s F 7 .CFtNF(82%)+2(C 5 H s ) 2 Fe+F- 

Hot copper 101 or dicumeneohromium 116 at room temperature converts per- 
fluoroalkyl-JV^jy^difluoroamines into nitriles, e.g., 

25° 
n.C 3 F 7 .CF 2 .NF 2 + 4(C 9 H 12 ) 2 Cr c<H<(c]f>) > n-C 3 F 7 .C;N(96%) + 4(C 9 H 12 ) 2 Cr+F- 

Perfluoro-2,3,4,5-tetrahydropyridine (XV) is formed in almost quanti- 
tative yield when perfluoro-iV-fluoropiperidine is bubbled through mercury 
at 320°. 117 Pyrolysis of the piperidine over mild steel at 400-600°/l atm, 
however, yields a mixture of perfluoro-2,3,4,5-tetrahydropyridine (40%), 
perfluoro-(^-methylpyrrolidine) (XVI) (10%), perfluoro-(n-butylidene- 
methylamine) (XVII) (10%), and pentafluoropyridine (6%). The isomeric 
products (XVI) and (XVII) are produced by thermal rearrangement of per- 
fluoro-iV-fluoropiperidine and are formed almost exclusively when the 
pyrolysis is carried out in platinum at 550 °/l atm: 11B 



F 



F 



FjC-^ X CF 2 
partial * i 



FgC-^ X CF 2 



F2C\ N /CF 2 



defluorination I I * ' 

F2CV ^CF 



thermal FaC GF " 



tneimai - 1 i 

rearrangement I I + CF 3 .[CF 2 ] 2 .CF: N .CF 3 



I 

CF 3 
(XVI) (XVII) 

132 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Perfluoro-JV-fluoromorpholine undergoes a parallel thermal rearrange- 
ment, 118 and both reactions are believed to involve a nitrene intermedi- 
ate (A) ; in neither case is the acyclic perfluoroimine produced by pyrolysis 
of the five-membered ring-contraction product : 



^X 



F 2 CK" X CF 2 Jtfj-^NJF, 



I "* k s M ^ CT t WO, J OF, 



u 



FjC- 



insertion - -. > 



CF S 
l,aW CF 3 .CF 2 .X.CF 2 .N:CF 2 



/.\ isomerization 

CF 3 .CF 2 .X.CF:N\CF 3 
(X = CF 2 orO) 

The first method of synthesis of pentafluoropyridine comprised pyrolysis 
of perfluoro-JT-nuoropiperidine over mild steel at low pressure (compared 
with reactions at atmospheric pressure, defluorination is encouraged 
through the increase in the proportion of molecule- wall collisions) : 119 

F 2 F 

FjC^ ^CFj „ FC- / ° V CF 

| j 2 Fe, 580-610°/ <1 mm y I / o 

I I contact time ca. 1 sec If I ^ 26% ^ 

FsC^ /CF 2 FC^ ^CF 



All organic JW-fluoro-compounds oxidise iodide ion to iodine, perfluoro- 
JT-fluoroamines proving no exception. 120 Thus, ^-fluorobistrifluoromethyl- 
amine, (CF 3 ) 2 NF, which is unaffected by concentrated sulphuric acid or 
50% aqueous potassium hydroxide at 20 °, readily liberates iodine from 
warm aqueous potassium iodide; similarly, perfluoro-jy-fluoromorpholine 
and perfluoro-jy-fluoropiperidine are inert towards aqueous sodium hy- 
droxide or hydrochloric acid at 80° but react rapidly with sodium iodide 
in aqueous acetone at 20° with near quantitative liberation of iodine, 
according to the equation : 

F 2 C /X ^CF 2 F 2 C /X ^CF 2 

I | +2I- + 4H a O— >- | | +I 2 + NH 4 F + 2HF + 2F- 

F2C\ N /CF 2 H0 2 C C0 2 H 



(X = O or CF 2 ) 

133 



Perfluoroalkyl Derivatives of the Elements 

The mechanism of this diagnostic test for the >N— F group in a perfluoro- 
amine is believed to involve attack by iodide on the fluorine of the N—F 
bond, 121 e.g., 



H H 

jL L (ii)-HF I I || 

>*kN> o^Nn/^o CO s H CO 



(B) 



C0 2 H C0 2 H 



+ NH 3 



H 



and, for example, the reduction of perfluoro-^T-fluoromorphoIine to per- 
fluoromorpholine with hydriodic acid (see p. 125) can be rationalized in 
terms of a proton-abstraction reaction involving nitranion (B). Other 
reactions of perfluoro-^-fluoromorpholine and perfluoro-JT'-fluoropiperidine 
are known which may be initiated by nucleophilic displacement on the 
fluorine of the N— F bond, 122 and reductive defluorination of perfluoro- 
JV-fluoroamines with ferrocene or dicumenechromium (see p. 132) may also 
involve the formation of transient perfluoronitranions, via one-electron 
transfer steps : 

^>C— NF 2 — ^ y<2— NF ► /C^-NF — -+ \c=*NF 

2. Perfluoro(methylenemethylamine), CF 3 .N:CF 2 . 

CFs-N O 

NO CTVCF,! 8 J Y 

CF,I >■ CFq-NO >■ 

3 u.v. light *"■»•"" 150 o | | 

CF 2 CF 2 
(ref. 123) 1 550°/5 mm Hg 
CF 8 .N:CF 2 (96%) + COF 2 
(ref. 124)J575° 

electrochemical , , ,„ „, , 

(CH 3 ) 2 N.COCl — flnorinatlon > (CF,) 2 N.COF(37%) 

134 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

This compound, b.p. -34° (c/. CF 3 .CF:CF 2 , b.p. -29°), has been studied 
in more detail than any other perfluoroimine. Called perfluoro-2-azapropene 
in American literature, it can be prepared fairly conveniently by either 
of the two methods shown above; unlike its hydrocarbon analogue, per- 
fluoro(methylenemethylamine) is monomeric and only feebly basic, if at 
all. In comparison with the C:C bond in perfluoropropene, the C:N bond 
in perfluoro(methylenemethylamine) is much more susceptible to nucleo- 
philic attack and considerably less prone to free-radical attack. like per- 
fluoropropene (see p. 34), the imine is attacked and dimerized by fluoride 
ion, which also catalyses the addition of carbonyl fluoride across the C:N 
bond; possible mechanisms for these reactions are given below. The facile 
hydrolysis of perfluoro(methylenemethylamine) was mentioned on p. 126. 



F- CF 2 =N— CF 3 „ * (CF 3 ) 2 N- 

r*t Pi v£\ 

(CF 3 )aN- CF2=N— CF 3 v (CF 3 ) 2 N— CF— N— CF 3 



F? 

0~ CK 



(CF 3 ) 2 N- G > (CF 3 )2N-C— F > (CF 3 ) 2 N-C^ +F 

<Z\. I F 



(CF 3 ) 2 N— CF=N— CF 3 + F- 

:0 



F F 



Some reactions of perfluoro(methylenemethylamine) are shown in Fig. 4.1 . 
The mercurial [(CF 3 ) 2 N] 2 Hg, which can also be prepared from cyanogen 
chloride and mercuric fluoride 127 but in much lower yield than from per- 
fluoro(methylenemethylamine) and mercuric fluoride, 128 and the derived 
JT-halogenoamines (CF 3 ) 2 NX (X = CI, Br, or I) are versatile reagents for 
the introduction of the (CF S ) 8 N— group into molecules, as illustrated by 
the examples in Fig. 4.2. The iV-halogenoamines are useful in synthesis 
on two accounts: when photolysed they decompose homolytically to yield 
(CFjJjN- radicals and halogen atoms, while under ionic conditions they 
function as 'positive' halogen compounds and act as sources of (CF 3 ) 2 N~. 
The latter type of reactivity accounts for the observations that J^-bromo- 
bistrifluoromethylamine is destroyed by aqueous potash : 



HO" Br— N(CF 3 ) 2 — > HOBr+CF 3 .N— CF 2 — F — »• CFs.NiCFj+F - 

OH-, H s 

COJT,NH 3 ,5.F- 



135 



Perfluoroalkyl Derivatives of the Elements 







o 



ft 
o 

o 
CO 



Si 



si 



I- 



o 



o- 






£ 



6 



ea 

o 






€ 



o 

ft 
o 



a/" 



09 

g 

O 

o 



I 
? 

■* 



136 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 




10 



137 



Perfluoroalkyl Derivatives of the Elements 

and reacts with silver cyanide to give perfluoro(methylenemethylamine), 
silver fluoride, and cyanogen bromide : 

'""> r> -^1 f — \ 

CN~ Br— N(CF 3 ) 2 — >■ BrCN+CF 3 .N— CF 2 — F* Ag + — ► CF 8 .N:CF 2 +AgF 

The great value of compounds of the type (CF 3 ) 2 MX (X = halogen other 
than F) as synthetic intermediates is common throughout Group V B 
(M = N, P, As, or Sb), but nucleophiles usually attack the central atom 
M in the cases of the phosphorus, arsenic, and antimony compounds (see 
pp. 149, 159). 

3. Trifluoronitrosomethaue. During the last decade the chemistry of poly- 
fluoronitrosoalkanes has received much attention, in both academic and 
industrial circles. Trifluoronitrosomethane has been studied in the most 
detail and some of its properties are briefly explored in this section. 

In the laboratory, trifluoronitrosomethane is best prepared either by 
photolysis of trifluoroiodomethane with nitric oxide in the presence of 
mercury (to absorb the iodine formed) 129 or by careful pyrolysis of tri- 
fluoroacetyl nitrite, 130 which can be obtained in almost quantitative yield 
from trifluoroacetic anhydride and dinitrogen trioxide : 

CF,I+NO n " T '" ght > CF„.NO(80%) 

Hg 
0° 100° 

(CF 3 .CO) 2 0+N 2 3 — ► CF 3 .CO.O.NO(97%) — — ► CF 3 .NO(59%)+CO, 

IS2 diluent 

The latter method has been adapted for the preparation of trifluoronitroso- 
methane on a small industrial scale in America; 131 the acyl nitrite, which 
explodes violently if its neat vapour is heated above 140° at atmospheric 
pressure, is decomposed by dripping it into boiling perfluoro(tributyl- 
amine). The above reactions exemplify the two most important general 
routes to perfluoronitrosoalkanes, viz., reaction of nitric oxide with per- 
fluoroalkyl radicals and pyrolysis or photolysis of perfluoroalkanoyl nitrites ; 
two less useful general methods are reaction of perfluoro-olefins with 
nitrosyl fluoride 132 and cleavage of perfluoroalkyl derivatives of metals or 
metalloids with nitrosyl chloride 133 (c/. p. 112). 

Trifluoronitrosomethane is a deep blue, toxic, monomelic gas, b.p. 
— 84-6°, which condenses to a deep blue, unassociated liquid. It does not 
form a colourless dimer of the di-JV-oxide type (XVIII ; R = CF 3 ) and 
it does not isomerize to the compound CF 2 :NOF. Few alkyl nitroso- 
compounds are monomelic under normal conditions, and four types have 

R \ S° 

(XVIII) 
138 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

been distinguished: 184 (a) Those which contain the CH.NO group form 
colourless dimers (XVIII; e.g., R = CH 3 ) or isomerize to an oxime [e.g., 
CH3NO - CH 2 :NOH; (CH 3 ) 2 CH.NO - (CH 3 ) 2 C:NOH]. (b) Those which 
contain hydrogen and halogen substituents on the a-carbon atom iso- 
merize only slowly, but form colourless dimers (e.g., CH 3 .CHCl.NO -* 
CH 3 .CCl:NOH or XVIII where R = CH 3 .CHC1). (c) Those in which the 
nitroso group is attached to a tertiary carbon atom cannot isomerize, but 
yield colourless dimers [e.g., (CH 8 ) 3 C.NO]. The dimers of these three types 
of nitroso-compound often partly dissociate to the blue monomelic forms 
when they are melted or dissolved in an organic solvent, (d) Per- and poly- 
fluoronitrosoalkanes, which do not form normal dimers. This has been 
attributed to the powerful inductive effect of the fluoroalkyl groups, which 
reduces the availability for bond formation of the lone pair of electrons 
of the nitrogen atom of the nitroso groups in these compounds and thereby 
prevents formation of dimers of type (XVIII). 

In the absence of light, pure trifluoronitrosomethane can be stored un- 
changed for years in dry glass vessels at room temperature; however, at 
100° it decomposes during several months to a mixture of the comppunds 
CF 3 .NO a , CF 3 .N:N.CF 3 , and CF 3 .N:CF 2 and nitrogen." 2 On exposure to 
light, and particularly to u.v. light, trifluoronitrosomethane is quantitatively 
converted into a brownish-red dimer. This has been shown to be 0-nitroso- 
bistrifluoromethylhydroxylamine, (CF 8 )gN.O.NO, and is thought to be 
formed by the following free-radical mechanism : i»»i*5 

CF..NO n - T -" 8ht > CF3. + NO 

CF 3 . + CF s .N:0 ► (CF 3 ) 2 N.O. 

(CF 3 ) 2 N.O.+CF 3 .N:0 >- (CF^N.O.NO+CF,. CT * N: °> etc. 

or (CF 8 ) 2 N.O.+NO >- (CF 3 ) 2 N.O.NO 

Reaction of the dimer with hydrochloric acid in the presence of mercury 
yields JV^-bistrifluoromethylhydroxylamine, 138 which can be oxidised 
with silver oxide 136 ' 137 or potassium permanganate 138 to bistrifluoromethyl- 
nitroxide, a purple gas that resists decomposition 139 when heated at 200° 
in glass for four hours : 

(CF 3 ) 2 N.O.NO ^5l> NOC1 (removed by Hg) + (CF s ) 2 N.OH(72%) 

Ag>0 85" 
v 
(CF„) 2 N. 0.(90%) 

Examples of isolable nitroxides (i.e., free radicals of type R 2 N— O- •«-» 
R 2 N— O) have been known for a long time and the discovery of bistri- 

10 * 139 



Perflicoroalkyl Derivatives of the Elements 

fluoromethylnitroxide and related polyfluoronitroxides has coincided with 
a resurgence of interest in such species. 140 Bistrifluoromethylnitroxide is a 
general reagent for the introduction of the (CF^gN.O — group into com- 
pounds, and its use has been exemplified in cases where the substrate is 
itself a free radical, contains an atom or group capable of undergoing 
radical abstraction, or has an unsaturated function that will partake in a 
radical-addition reaction, e.g., 137 

C 6 H 5 .CH 3 + (CF 3 ) 2 N.O. -^ (CF s ) a N.OH + C 6 H 5 .CH 2 . (CT>)slf -°- 



C„H 5 .CH 2 .O.N(CF 3 ) 2 (74%) 

23° 

CH 2 :CH 2 +2(CF 3 ) 2 N.O- > (CF 3 ) a N.O.CH 2 .CH 2 .O.N(CF 3 ) 2 (99%) 

23° 

CF 3 .CF:CF 2 + 2(CF 3 ) 2 N.O. » (CF 3 ) a N.O.CF 2 .CF(CF 3 ).O.N(CF s ) 2 (98%) 

°" 

23° J 

CFjs.^O-MCFaJaN.O- > CF 3 .N-O.N(CF 3 ) 2 — > CF 3 .N0 2 (98%) + (CF 3 ) 2 N- 

(CF 3 ) 2 N.O.N(CF 8 ) 2 (99%) 

Other methods for the introduction of the (CF 3 ) 2 N.O — group include 
treatment of reactive halides with the sodium [from (CF 3 )gN.OH and 
NaOH 141a ] or mercury [from (CF s ) 2 N.O- and Hg 141b ] derivative of bistri- 
fluoromethylhydroxylamine, e.g., 

. !±5^ >. (CF 3 ) 2 N.O.CH 3 (82%) 

(CF 3 ) 2 N.O-Na+ -^~ (CH,),81C1 > (CF 3 ) 2 N.O.Si(CH 3 ) 3 (47%) 

\ CF..COC1 
\ - >- (CF 3 ) 2 NO.CO.CF 3 (88%) 

5^ > [(CF 3 ) 2 N.O] 3 B(100%) 

[(CF 3 ) 2 N.O] 2 Hg -/ pc , 

X > [(CF 3 ) a N.O] 5 P(74%) 

Polymer scientists are currently interested in trifluoronitrosomethane 
because it combines with certain fluoro-olefins to yield novel 1 : 1 alternating 
copolymers which may prove useful for military and other special applica- 
tions. 131 The N=0 bond acts like a C=C bond, so that the polymers, known 
as nitroso polymers (those with elastomeric properties are called nitroso 
rubbers), are characterized by the — N— O — C — C — backbone. 

The discovery that trifluoronitrosomethane, which has not yet been 
found to undergo homopolymerization, will copolymerize with certain 
fluoro-olefins was made by Barr and Haszeldine in the mid-1950's. Initially, 

140 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

they found that the nitroso-compound combines readily with tetrafluoro- 
ethylene to yield an oxazetidine (XIX) and a 1:1 copolymer (XX), and 
that low reaction temperatures favour formation of the polymer : 128a 



20° 



CF,— N=0 + CF S =CF S - 



CF 8 — N O 

-> | | (62%) 

(XIX) 



-45° to 20° 



-> [— N— O— CF 2 — CF a — ]»(64%) 

CF S 

(XX) 



Both products are chemically very stable, but can be degraded at elevated 
temperatures to an equimolar mixture of perfluoro(methylenemethylamine) 
and carbonyl fluoride in almost quantitative yield. This degradation 
probably proceeds by fission of the N— bond, followed by elimination of 
carbonyl fluoride from the resultant perfluoroalkoxy radical (see p. 86), 
e.g., 



— CF 2 — N— 
CF S 



O— CF 2 — CF 2 — N— O— CF 2 — CF 2 — N— O— CF 2 — 
CF 3 CF 3 

400°/5 mm Hg 



— CF 2 — N- + -0— CF 2 - 
CF 3 



-CFjj— N— O— CF 2 — 0F 2 — N— O— CF a - 
CF« CF« 



COF 2 + -CFj— N— 
CF. 



— O— CF 2 — CF 2 — N— O— CF 2 - 
CF. 



CF 2 =N + -O— CF 4 - 
CF S 



-CFj— N— O— CFjt 
CF. 



etc. 



141 



Perfluoroalkyl Derivatives of the Elements , 

Commercially, the copolymer of trifluoronitrosomethane with tetra- 
fluoroethylene (often referred to as simply nitroso rubber) is prepared 
using an aqueous suspension polymerization technique at — 25° ; 131 no 
initiator is required, and although it seems certain that a free-radical 
mechanism is operative doubt still exists as to the exact nature of 
the initiation process and the reason for the strict alternation of 
monomer residues in the polymer chain. As prepared by the above 
method, nitroso rubber is a colourless, translucent, high-molecular- 
weight (ca. 1-3 x 10 6 ) material that has excellent resistance to chemi- 
cal attack and hydrogen-containing solvents and shows good low-tem- 
perature flexibility (at —31° it is only twice as stiff as it is at 25°, 
and its glass transition temperature is —51°), presumably owing to 
the ease of rotation of the chains about the N— links and to low 
inter-chain attractive forces and the presence of the relatively bulky 
pendent CF 3 groups, which hinder crystallization. End-uses for the co- 
polymer will be based on the outstanding combination of these pro- 
perties and not to any great extent on its thermal stability since the 
upper limit for continuous service in air is 162°; above 200° decom- 
position of the copolymer according to the scheme given above is readily 
detected, but it should be noted that it is completely non-flammable 
since neither perfluoro(methylenemethylamine) nor carbonyl fluoride 
will support combustion. Unfortunately no satisfactory method for the 
vulcanization of nitroso rubber has been developed yet. The difficulty 
of introducing links between chains stems from the chemical inertness 
of the system, which is a most desirable property and must be main- 
tained. Vulcanization of nitroso rubber is believed to occur when it 
is heated (ca. 120°) with polyfunctional amines (a mixture of hexame- 
thylene diamine and triethylene tetramine is preferred), but obviously 
much unwanted degradation also occurs since the vulcanizate has 
poor tensile strength and is less stable, both thermally and chemically, 
than the raw copolymer. Efforts are now being made to introduce 
structural features into the basic nitroso-rubber chain that will enable 
cross links to be inserted under controlled conditions; for example, ter- 
polymers known as carboxyl-nitroso rubbers have been prepared from 
trifluoronitrosomethane, tetrafluoroethylene, and perfluoro-y-nitrosobutyric 
acid (ON.CF a .CF 2 .CF 2 .C0 2 H; 0-5 to 2-0 mole % of this monomer is used) 
and methods of vulcanization involving pendent acid groups on adjacent 
chains are being investigated. 131,144 

The nitroso function in trifluoronitrosomethane is also susceptible to 
nucleophilic attack. 145 Thus although the nitroso-compound is stable to 
aqueous acid, it is rapidly decomposed by dilute alkali with the formation 
of trifluoronitromethane (which is best prepared by conventional oxidation 
of trifluoronitrosomethane 134 ), hexafluoroazoxymethane, and fluoride, car- 
bonate, and nitrite ions, 143 possibly as follows : 14a 

142 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 



r° °" e+ a- 0_ 

^Vll I CF a .N=0 I Ci 

HO" *N.CF 3 — > HO.N.CF3 *+ CF 8 N-rO— N.CF 3 

6+ -C? I 

OH 



-F- OH-HsO • _ . , 

— > CF 2 :N.OH »- F - ,CO| ,ete. 



?" 

— . /CF..N— O I -OH- +,— v 

CF 8 .N0 2 +CF s .N.OH 4 >-. CFs.Nt-KT.CFs >■ CF 3 .N(0):N.CF 3 

•J I 

^OH 
^°-^ CF,.K: 0F ' jro 

Treatment of trifluoronitrosomethane with hydrogen iodide in ether yields 
■ST-trinuoromethylhydroxylamine, which condenses with the nitroso-com- 
pound to yield hexafluoroazoxymethane : 

CF 3 .N:0+HI— »CF 8 .NI.OH HI > I 2 +CF 3 .NH.OH CF '- If0 > CF 3 .N(6):N.CF 3 

— HjO 

Primary aliphatic amines attack trifluoronitrosomethane at low temper- 
atures to give azo-compounds, e.g., 146 

-j on f n ono — + 

CF 3 .N:0+CH 3 :NH 2 > CF 3 .N(0).NH 2 .CH 3 — > 

CF 3 .N(OH).KTH.CH 3 ~ H '°> CF 3 .N:N.CH 3 (57%) 

and analogous reactions occur between primary aromatic amines or organic 
derivatives of hydrazine and the nitroso-compound. 145 The phosphorane 
(C a H 5 ) 3 P:CF 2 is claimed 77b to be formed when the product of the reaction 
between trifluoronitrosomethane and ammonia in ether at — 70° is treated 
with triphenylphosphine ; difluorodiazomethane, otherwise unreported, is 
postulated as an intermediate : 

CF 3 .N:0+NH 3 —5^1 CF 3 .N:NH — 5> CF 2 N 2 (C ' Ht)8P > (C 6 H 5 ) 3 P:CF 2 

— Nj 

4. Hexafluoroazomethane, CF 3 .N:N.CF 3 . This compound, which normally 
exists as the trans form, is a colourless gas that condenses to a pale green 
liquid, b.p. —32°; it is usually prepared by one of the following meth- 
ods : s 8 - 90 - 147 - 14 s 

IF 5 , ca. 140° 
ICN ' autoclave \ < ca - 50% ) 

Br 2 C:N.N:CBr 2 *%££ ^ ► CF 3 .N:N.CF 3 (92%) 

C1CN ^•f / (90%) 



flow system 



143 



Perfiuoroalkyl Derivatives of the Elements 

The last type of reaction is used to prepare higher members of the per- 
fiuoroazoalkane series, 149 ' 160 e.g., 

CP °- CN "SiS <W,N:N.C 2 F 6 (90%) 

and when applied to cyanogen and perfluoromalononitrile provides the 

i 1 i 1 

cyclic azo-compounds CF 2 .N:N.CF a 92 and CF 2 .N:N.CF 2 .CF 2 , 161 respec- 
tively. 

When irradiated with ultraviolet light or pyrolysed, hexafluoroazo- 
methane decomposes homolytically in conventional fashion and is thus a 
useful source of trifluoromethyl radicals. 89 ' 152 ' 1M Photolysis of neathexa- 
fluoroazomethane yields nitrogen, hexafluoroethane, tetrakistrifluoro- 
methylhydrazine, and hexakistrifluoromethyltetrazine ; the relative amounts 
of these products vary with pressure, the last two being produced more 
abundantly as the pressure is increased. 162 The same products are formed 
slowly when the azo-compound is kept at 270°, 1M and the same decomposi- 
tion mechanism has been proposed as for the photolytic reaction, viz., 

CF 3 .N:H.CF 3 *"**, > 2CF 3 .+N 2 

or u.v. light 3 a 



2CF,. 



.N:N.CF 3 y (CF 3 ) 2 N.N.CF 3 

CF 3 . + (CF 3 ) a ]Sr.N.CF 3 > (CF 3 ) 2 N.N(CF 3 ) 2 

2(CF 3 ) 2 ST.N.CF 3 > (CF 3 ) 2 N.N(CF 3 ).N(CF 3 ).N(CF,) 2 

The N— N bond in tetrakistrifluoromethylhydrazine is surprisingly strong 
[only 16% decomposition to (CFgJjN- radicals occurs when the compound 
is passed through a nickel tube at 550° with a contact time of eleven min- 
utes; 89 c/. N 8 F 4 , p. 129] and, paradoxically, it appears that this link is 
strengthened and shortened by virtue of strong non-bonded repulsions 
across it. As revealed by electron diffraction measurements, 166 repulsive 
forces operating between the CF 8 groups cause the two >N— pyramids to 
flatten almost to planarity and to set the dihedral angle between opposite 
tCF 3 ) 2 N— groups at a value of ca. 90°; however, the N— N bond length is 
shorter by ca. 0-05 A than its counterpart in hydrazine. A molecular orbital 
description of the molecule suggests that the short N— N bond may be 
understood in terms of the enhancement of ^-bonding ensuing from the 
nearly D 2d symmetry imposed by steric forces. 155 

144 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Hydrogenation of fraras-hexafluoroazomethane yields .W,.y'-bistrifluoro- 
methylhydrazine, which gives the cis-foxm. of the parentazo-compound 
when oxidised with permanganate r 80 - 145 

F » C \ H», Pd/C 

N:N \ CF 20°, autoclave* CF 3 .NH.NH.CF S (96%) 



KMnOj, acetic acid/anhydride, 20° 

/CF S 
N:N X (60%) 

The hydrazine can also be prepared by hydrofluorination of tetrafluoro- 
formaldazine (pernuoro-2,3-diazabuta-l,3-diene), a compound which, unlike 
its hydrocarbon analogue, is stable at room temperature and which yields 
oUfluoromethylenimino radicals (CF 2 :N->, nitrogen, and difiuorocarbene 
when irradiated with ultraviolet light. 147 - 156 

AgF HF, 100" 

-*■ CF 2 :N.N:CF 2 (33%) ( > CF 3 .NH.NH.CF 3 




125 KF, heat 



5. Difluorodiazirine (CF 2 .N:N),BistrifluoromethyldiazMne [(CF 8 ) 2 C.N:N], 
and Bistrifluoromethyldiazomethane [(CF 3 ) 2 C.N 2 ]. Perfluorodiazo-com- 
pounds have been added to the list of known types of fluorocarbon deriva- 
tive only recently. Bistrifluoromethyldiazomethane, a pale yellow liquid, 
b.p. 13°, is the simplest perfluorodiazoalkane known (see p. 143 for in- 
formation about difluorodiazomethane) ; it is prepared from hexafluoro- 
acetone as follows : 187 

«*■>■«- 'Tr.ts^rr- <cr,,,o iK H„.%, «^ 

(CFs^CJNHgJ.NH.NH, P '°" heat > (CF 3 ) 2 C:N.NH,(68%) 



(not isolated) 



Pb(O.0O.CH>)4, 0-25° 

_> (CF 3 ) 2 C.N 2 (77%) 



benzonltiile 



Acetic acid, formed as the last step in the synthesis is proceeding [(CF s ) a C : 
N.NH 2 + Pb(O.CO.CH 3 ) 4 - (CF 8 ) a CN 2 + Pb(O.CO.CH 3 ) 2 + 2GH a .COaH], 
does not decompose the diazo-compound, whereas non-fluorinated diazo- 
methanes are commonly used to esterify organic acids. Bistrifluoromethyl- 
diazomethane is neither impact nor static sensitive and can be stored un- 
changed for long periods at — 78°; however, it decomposes slowly at room 
temperature and more rapidly when heated or irradiated with light to give 
nitrogen and bistrifluoromethylcarbene. The carbene will rearrange to per- 

145 



Perfluoroaikyl Derivatives of the Elements 

fluoropropene or attack undissociated bistrifluoromethyldiazomethane to 
yield perfluoro(tetramethylethylene) and nitrogen unless a suitable trap 
is present: 167 

430°/035 mmEg "\ , l, . 

(CF 3 ) 2 C.N- 2 '— ~ > CFs.CiCFs — ► CF 3 .CF:CF 2 92%) 

■ z ■* flow pyrolysis over silica " t« 5 

F 

(CF 8)l! C.N s /heUum ^ J^^ glllca > (CF 3 ) 2 C:C(CF 3 ) a + CF,CF:CF a 

(ca. 50 : 50 mixture ; 80 % total yield) 



an excess of benzene / \/CF 3 




CH(CF 3 ) 



«*■>.<>•* ,00,,^^ > ( X cf (62%) + J) I (8%) 



Bistrifluoromethylcarbene (apparently in the singlet state) can also be 
generated by thermolysis of bistrifluoromethyldiazirine, a colourless liquid, 
b.p. — 12°, which can be stored safely in a steel cylinder at room tempera- 
ture : 1B7 

,„„ » „A, U ,. 350°/latm. 

(CF 3 ) 2 C. /helium- — — — -— >■ 

\ » flow pyrolysis in silica 

1,2-F shift 
/ > CF 3 .CF:CF 2 



N 2 + (CF 3 ) 2 C: — ( 1 1 

\ (CF 3 ) a C.N:N , 
> (CF 3 ) 2 C:N.N:C(CF 3 ) 2 



(CF 3 ) 
Cs 



3)1 



H 



N N H x X CH 3 H x X CH 2 .CH(CF 3 ) 2 

(major product) 

H 3 Cv /CHfCF,,);. 

+ >=\ 

H x X CH 3 

(traces) 

Like bistrifluoromethyldiazomethane, bistrifluoromethyldiazirine can be 
prepared from hexafluoroacetone : 157 

(see above) , , HN a ,0-10° / * 

(CF 3 ) 2 CO , (CF 3 ) 2 C:NH (70%) -^^—^ (CF 8 ) 2 c( (56%) 

boil (85°) A 1 * .„ „,, Pb(O.CO.CH,) 4 , 25-35° , , /^ , 
► (OF 3 ) 2 C I (67%) —-^^ > (0F 3 ) 2 C( || (87%) 

146 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Several other polyfluorinated diazirines are known besides bistrifluoro- 
methyldiazirine. 168 ' 159 These include difluorodiazirine, b.p. —91°, the most 
extensively studied compound of its class, which can be prepared by reduc- 
tive defluorination (c/. p. 132) of bis(difluoroamino)difluoromethane : 158, 16 " 



^/ a ferrocene, C,H 4 (Cg,),, 25° 

*\ or (C 4 H,) 4 N+I-. CH,CN, 25° ' 

N NF. 



X NF 



ferrocene / ,, 

X N 



(see Table 4.3, p. 128) (possible intermediate) 

or by fluoride-catalysed rearrangement of difluorocyanamide obtained by 
direct fluorination of phosphate-buffered aqueous cyanamide : M1 

H 2 N.CNaq. ^$- F 2 N.CN(~20%) -55L* F 2 (/ || (-100%) 

X N 

In the absence of light, difluorodiazirine is much less reactive than either 
diazirine or diazomethane ; it can be stored in glass and does not react 
with mercury, hydrocarbon stopcock grease, boron trifluoride, strong acids 
(e.g., CF 3 .C0 2 H), or strong bases. 182,163 However, it dissociates smoothly 
when photolysed or pyrolysed to yield nitrogen and singlet (apparently) 
difluorocarbene, 184 e.g., 

F 2 

««-but-2-ene \ / n / 

,C < (83%) 



, / 175-185° / \ 

/[ H 3 C CH 3 

F 2 C: II — <- CF,.C0,H > CF co CHF (7g0/o) 

x " \ u.v. light 



II 



F 2 

\ 
> F 2 C C' (66%) 



CF 2 :CF.CN _ / \ / F 



u.v. light 

and is readily attacked by organo-derivatives of tervalent phosphorus, 
possibly according to the mechanism shown below, with the formation of 
JT'-cyanophosphorus imides, 166 * e.g., 

F «P\J + (CoHsJsP ^ > (C 6 H 6 ) 3 P:K.CN(88%)+(C 6 H 5 ) 3 PF 2 (87%) 



B 3 P |^CF-F^ S^-N<J|V _► -Rp^— C=N+B 3 PF 2 

147 



Perfluoroalkyl Derivatives of the Elements 

6. Trifluoromethyl Isoeyanide, CF 3 .NC. This compound, a gas, b.p. - 84 
to —83°, is the only perfluoroalkyl isoeyanide known. It can be prepared 
by treatment of the amine CF 3 .NH.CF 2 Br with magnesium 165 b or from 
trifluoronitrosomethane, 1850 as shown below, and must be stored at low 
temperatures otherwise it polymerises to a yellow solid that possibly has 
the structure [— C(:N.CF 3 ).C(:N.CF„)— ]„. like alkyl isocyanides, tri- 
fluoromethyl isoeyanide is converted into the corresponding cyanide and 
cyanate, respectively, by heat or treatment with mercuric oxide. 

CF 8 .NO + CF 2 :CF.COF -^l* [_N(CF 3 ).O.CF 2 .CF(COF)-]„ - 560 ° /10 '' mm Hg > 

COF 2 + CF 3 .N:CF.COF — — -> CF..NC + COF, 

a 450°/2 mm Hg s 2 

B. Phosphorus 

So far only trifluoromethyl and heptefluoro-n-propyl derivatives of 
phosphorus have received detailed attention, a considerable amount of 
information on the former being available. The following is a fairly brief 
account of the basic chemistry of the trifluoromethyl compounds. 

1. Synthesis. Trifluoroiodomethane reacts with either red or white phos- 
phorus at temperatures in the range 195-230° and under pressure to yield 
a mixture of tristrifluoromethylphosphine, (CF 3 ) 3 P, iodobistrifluoromethyl- 
phosphine, (CF 3 ) 2 PI, and di-iodotrifluoromethylphosphine, CF 3 .PI 2 . 166 This 
reaction is of great importance because nearly all other trifluoromethyl 
derivatives of phosphorus are derived from the three products named 
(see Fig. 4.3). Rather surprisingly, no trisheptafluoro-n-propylphosphine, 
(n-C 3 F 7 ) 3 P, is formed when heptafluoro-1-iodopropane is heated under 
pressure with phosphorus, but only a mixture of the iodophospbines 
(n-C 3 F 7 ) 2 PI and n-C 3 F 7 .PI 2 . Like their trifluoromethyl analogues, the 
heptafluoropropyliodophosphines can be readily converted into other 
derivatives. 167 

A convenient route to the mixed methyl-trifluoromethyl compounds 
(CF 3 )„P(CH 3 ) S _ M lies in the exchange reaction between tristrifluoromethyl- 
phosphine and methyl iodide at elevated temperatures : M8 

CH" T ptr t f 1 TT T 

(CF S ) 3 P ~~> (CF^P.CH, — ^> CF 8 .P(CH 8 ) 2 -r-^ [CF 8 .P(CH,) 8 ] + I- 



4- 



240° 



240° 



fast 



1 P(CH 3 ) 3 



CT,I, no reaction CFjI, no reaction CF»I, 20" 

2. Properties and Reactions of Some Trifluoromethyl Derivatives of Phos- 
phorus, a. Boiling Points. The boiling points of some trifluoromethyl deriv- 
atives of phosphorus, together with those of the analogous nitrogen, arsenic, 
antimony, and bismuth compounds, if known, are listed in Table 4.5. As 
might be expected, it is found that in most series of compounds of the type 

148 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 



O 



<£ 


^9 


>o 


O 


e» 


» 






o 


Ph 


6 


©4 


Ph 


09 


- W 


O 



fc ~ 
B 4 






3 



o 
o 



o 

Ph 



o 






Ph 

©1 


3 


"i 


F4 


f? 









O 







K 


ft 
Ph 


W 




Ph 


« 


^ 


PU 


£ 




O 



pu 
o 



Ph 




W 
o 

o 

Ph 

*» 



P4' 

6 

Pi 

o 



w 

o 

Pi 
O 



5? 

■* 

(N 

nT 

Ph 

r" 



03 





W 


ol 


g 


O 


O 


Ph 


w 


PR 


Pk 


O 


^ M 






M 
o 

o 
W 

Ph 
o 



a. 



fe* 


ri* 


(±i Ph 








3 O 


I 


1 


1 1 


Ph- 


— Ph 


Ph Ph 




>-? 



fe 
o 



© 



149 



Perfluoroalkyl Derivatives of the Elements 

shown in the table the boiling points of the members increase appreciably 
as the atomic weight of the Group V atom increases. In general, the tri- 
fluoromethyl compounds have appreciably lower boiling points than their 
methyl analogues [e.g., cf. (CH 3 ) 3 N, b.p. 3-5° and (CF 3 ) 3 N, b.p. -11°; 
(CH 3 ) 3 P, b.p. 37-8° and (CF 3 ) 3 P, b.p. 17-3°; (CH 3 ) 3 As, b.p. 51-9° and 

Table 4.5. Boiling Points (°C) ol Some Trifluoromethyl DeriratiTes of the Group 

Vb Elements 



Compound M = 


N 


P 


As 


Sb 


Bi 


CFj-MHss 





— 26-5 


— 12-5 


— 


— 


CFj.MClg 


— 


37 


71 


— 


— 


CFsMIa 


— 


69/29 mm 


100/48 mm 


200 


— 


CFj.MJCH,^ 


— 


46-8 


58 


85-8 


121 


(CF 8 ) a MH 


— 6 


1 


19 


— 


— 


(CF S ) 2 MF 


— 37 


— 11-8 


25 


— 


— 


(CF 3 ) 2 MC1 


— 9 


21 


46 


ca. 88 


— 


(CF,) 2 MI 


57 


72-73 


92 


16/8 mm 


— 


(CF 3 ) 2 M.CH 3 


— 


35-2 


52 


— 


132 


(CF S ) S M 


— 11 


17-3 


33-3 


71-7 


— 


(CF 3 ) 3 MF 2 


— 


20 


57-58 


— 


— 


(CF 3 ) 2 MC1 3 


— 


82/355 mm 


93-95/722 mm 


13/5 mm 


— 


(CF 3 ) 3 MC1 2 


— 


107 


98-5 


101 





(CF 3 ) 3 As, b.p. 33-3°] ; however, a rise in boiling point accompanies sub- 
stitution of one or two trifluoromethyl groups for methyl groups in tri- 
methylbismuth (b.p. 110°), which is contrary to the cases of the corre- 
sponding phosphorus, arsenic, and antimony compounds, where the boiling 
points rise with the replacement of one CH 3 group in (CH 3 ) 3 M and then 
fall through (CF 3 ) a M.CH 3 to (CF 3 ),,M. 

b. Tristriflttoromethylphosphine. Tristrifluoromethylphosphine is a water- 
stable, colourless liquid which boils some 20° below its methyl analogue, 
and is spontaneously inflammable in air. The influence of the strongly 
electronegative trifluoromethyl groups is to be seen in the absence of com- 
pound formation between tristrifluoromethylphosphine and sulphur, carbon 
disulphide, silver iodide, mercuric iodide, boron trifhioride, or platinous 
chloride. 166, 169 The trifluoromethyl group acts like a pseudohalogen in this 
respect and tends to reduce considerably the donor properties of a phos- 
phorus atom to which it is attached. 

Tervalent phosphorus compounds PX 3 (X = any group) can form co- 
ordination compounds with electron acceptor molecules which have filled 
or partially filled d orbitals. If the d orbitals of the acceptor are filled, the 
stability of the compound is governed mainly by the basicity of the tervalent 
phosphorus compound. When the phosphorus atom carries strongly electro- 

150 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

negative groups, as, for example, in the trihalides PF 3 and PCl 3 , either no 
or very unstable addition compounds are formed with strong acceptors 
like boron trifluoride. If quaternary salt formation is taken as a rough 
guide to the basicity of the phosphorus atoms in different trifluoromethyl 
compounds, only dimethyltrifluoromethylphosphine will form complexes 
with electron acceptors which have no electrons in their d orbitals. This 
suggestion is supported by the isolation of an addition compound between 
dimethyltrifluoromethylphosphine and boron trifluoride and the failure to 
obtain similar compounds from tristrifluoromethylphosphine or methyl- 
bistrifluoromethylphosphine. 1 * 9 

If the acceptor has available d electrons, the bond formed has two com- 
ponents, the normal (x-bond formed by donation of the lone-pair electrons 
and the re-bond formed by donation of d electrons by the acceptor to the 
empty d orbitals of the phosphorus atom. An increase in the electronega- 
tivity of the groups attached to the phosphorus atom results in a decrease 
in the coordinate <r-bond strength, but this is offset to some extent by a 
strengthening of the w-bond. Thus, tristrifluoromethylphosphine reacts 
with nickel tetracarbonyl at room temperature to yield a mixture of the 
compounds (CF s ) 3 P.Ni(CO)3 and [(CFjJjjPJjjNifCO^ in a manner analogous 
to the replacement of carbon monoxide from nickel tetracarbonyl by 
phosphorus trihalides. 170 

In contrast to trimethylphosphine, which is easily converted into its 
oxide by air, tristrifluoromethylphosphine is rapidly decomposed by oxygen 
and the oxide (CF 3 ) 3 PO is not formed; however, it can be obtained almost 
quantitatively from tristrifluoromethylphosphine and dinitrogen tetroxide 
at room temperature 171 or in 70% yield by heating the phosphorane 
(CF 3 ) 3 PC1 8 , prepared from tristrifluoromethylphosphine and chlorine at 
— 45 °, with anhydrous oxalic acid. 172 Similarly, tristrifluoromethylphosphine 
sulphide cannot be prepared directly from tristrifluoromethylphosphine and 
sulphur (see above) but can be obtained from dichlorotristrifluoromethyl- 
phosphorane : 173 

AgjS 

> (CF 8 ) 3 PS + AgCl 



/ 20-60° 
(CF,),PC1 8 — <■ 

S~" (CF 8 ),PS+HC1 

In contrast to alkyl and aryl tertiary phosphine oxides, tristrifluoromethyl- 
phosphine oxide does not form a stable hydrate but is destroyed by water 
with the liberation of fluoroform and the production of bistrifluoromethyl- 
phosphinic acid, (CF 3 ) a PO(OH). Treatment of tristrifluoromethylphosphine 
oxide with dimethylamine yields fluoroform and the phosphmic amide 
(CF 3 ) 2 P(0).N(CH 3 ) 2 quantitatively; reaction of the latter product with 
anhydrous hydrogen chloride provides the phosphmic chloride 
(CF 3 )gP(0)Cl. 171 Tristrifluoromethylphosphine itself, although stable to 

151 



Perfluoroalhyl Derivatives of the Elements 

water, is quantitatively converted into fluoroform by aqueous alkali 
through a series of reactions which involve bistrifluoromethylphosphinous 
and trifluoromethylphosphonous acids [(CF 3 ) 2 P.OH and CF 3 .PHO(OH), 
respectively] as intermediates. 174 

Fluorination of tristrifluoromethylphosphine with sulphur tetrafluoride 
at 25° yields the phosphorane (CF 3 ) 3 PF 2 quantitatively. 176 This phos- 
phorane, b.p. 20°, is a good gas-phase source of difluorocarbene, since it 
undergoes facile thermal decomposition via a series of a-fluorine shifts 
from carbon to phosphorus, according to the scheme : 

(CF 3 ) 3 PF a ^» (CF 3 ) 2 PF 3 +:CF 2 
(CF 3 ) 2 PF 3 5=* CF S .PF < + :CF 2 
CF 3 .PF 4 ^ PF 5 +:CF 2 

In the absence of other reagents the difluorocarbene disappears irreversibly 
as tetrafluoroethylene, perfluorocyclopropane, and the polymer [ — CF 2 — ]„; 
in the presence of iodine and hydrogen chloride the products CF 2 I 2 and 
CHF 2 C1, respectively, are formed. The half-life of difluorotristrifluoro- 
methylphosphorane is 6 months at 60° and 12 hours at 100°; at the latter 
temperature the difluorocarbene reduces phosphorus oxyfluoride to phos- 
phorus trifluoride and is converted into carbonyl fluoride. 176 

The electrolytic behaviour of the chlorotrifluoromethylphosphoranes 
(CF 3 ) 2 PC1 S [from (CF 3 ) 2 PC1 and Cl 2 ] and (CF 3 ) 3 PC1 2 [from (CF 3 ) 3 P and Cl 2 ] 
is interesting: the former is a non-conductor in acetonitrile solution, while 
the latter forms conducting solutions in this solvent, a fact that has been 
attributed to the following ionization: 

2(CF 3 ) 3 PC1 2 ,=* [(CF 3 ) 3 PC1]+ + [(CF 3 ) 3 PC1 3 ]- 

which is analogous to the behaviour of phosphorus pentachloride : 

2PCl s ^=± PC1 4 + + PC1 6 - 

A reason for this difference in electrolytic properties has been proposed on 
the basis of the results of a determination of the structures of the two 
phosphoranes by i.r. spectroscopy, and by application of the postulate that 
a phosphorus pentahalide ionizes in a suitable medium via heterolytic 
fission of an apical bond in the trigonal bipyramidal molecules. 177 The 
phosphorane (CF 3 ) 2 PC1 3 appears to have a symmetrical bipyramidal 
structure in which the apices are occupied by trifluorometbyl groups, and 
its non-ionic behaviour is in keeping with the fact that the CF 3 _ ion has 
not been observed in any stable system so far investigated. Infrared anal- 
ysis indicates that the phosphorane (CF 3 ) 3 PC1 2 exists in a mixture of con- 

152 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

figurations, at least one of which contains an apical P— CI bond which is 
the site of ionization. 

The phosphorane (CF 3 ) 3 PI 2 is unknown; iodine reacts quantitatively 
with tristrifluoromethylphosphine at temperatures above 100° to give a 
mixture of the compounds (CF 3 ) 2 PI, CF 3 .PI 2 , CF 3 I, and PI 3 . 

c. Compounds Containing P—P Bonds. An interesting coupling reaction 
occurs when iodobistrifluoromethylphosphine is shaken with mercury 
at room temperature, and tetrakistrifluoromethyldiphosphine, (CF 3 ) 2 P. 
P(CF 3 ) 2 , is formed quantitatively." 6 Pyrolysis of this diphosphine at 350° 
yields the novel cyclic polymers tetralristrifluoromethylcyclotetraphos- 
phine (XXI) and pentakistrifluoromethylcyclopentaphosphine (XXII), 
which are best prepared by the action of mercury on di-iodotrifluoromethyl- 
phosphine at room temperature : 178, 179 

H CF 3 — P— P— CF 3 CF 3 — P P— CF 3 

CF 8 .PI 2 > I I (60%) + I J (40%) 

CF 3 — P— P— CF 3 CF 3 — P P— CF 3 

I 

CF 3 
(XXI) (XXII) 

The tetramer (XXI), which has a similar structure to phosphobenzene, 
(C g H s ) 4 P 4 , is a colourless solid, m.p. 66-3°, b.p. 145°, which is soluble in 
common organic solvents and inflames spontaneously in air to yield carbonyl 
fluoride, phosphoryl fluoride, and a solid residue containing phosphorus. 
Controlled oxidation of a solution of the tetramer in the fluorocarbon ether 
C 8 F 18 at 0-22° leads to the formation of polymeric anhydrides, (CF 3 .P0 2 ) M , 
which combine vigorously with water, in a manner reminiscent of P 2 5 , to 
yield trifluoromethylphosphonic acid, CF 3 .PO(OH) 2 , and polyphosphonic 
acids such as trifluoromethyldisphosphonic acid, [CF 3 .PO(OH)] 2 0. 

The tetramer is apparently insoluble in and unaffected by water at room 
temperature, but at elevated temperatures hydrolysis occurs with formation 
of the diphosphine (CF 3 .PH) 2 : 

(CF 3 ) 4 P 4 H, °' 140 "> (CF 3 .PH) 2 , CF 3 .PH 2 , HPO(OH) 2 , CHF 3 

Chlorine readily cleaves the P—P bonds of the tetramer, and at low tem- 
peratures the tetrachloro-compound CF 3 .PC1 4 is formed quantitatively; 
iodine attacks the tetramer at room temperature and converts it into the 
di-iodideCF 3 .PI 2 . 

Pentakistrifluoromethylcyclopentaphosphine (XXII, the pentamer), m.p. 
— 33°, b.p. 190°, which has been shown by X-ray crystallography to have 
a five-membered ring of phosphorus atoms widely distorted from a planar 
configuration, 180 also reacts quantitatively with chlorine and iodine to 

11 153 



Perfluoroallcyl Derivatives of the Elements 

yield the compounds CF 8 .PC1 4 and CF S .PI 2 respectively. It reacts with 
water to yield the first known triphosphine, H 2 (CF 8 .P) 8 : 

H«0. dislyme 

(CF 3 ) 6 P 5 '-t-fir- ► H 2 (CF 3 .P) 3 , (CF 3 .PH) 2 , CF 3 .PH 2 

and at temperatures above 250° undergoes thermal decomposition to 
yield mainly the tetramer, which is stable up to 300° : 

255° 

(CF 8 ) 6 P 5 » (CF 3 ) 4 P 4 (56%), (CF 3 ) 8 P, (CF.) 4 P 2 , P 4 

The high thermal stability of the tetramer and pentamer is associated with 
the supplementation of the P — P a-bonding by ji-bonds involving inter- 
action of the lone electron pairs on the phosphorus atoms with neighbouring 
phosphorus 3d orbitals. 178 

The open-chain triphosphine (CF 8 ) 2 P.P(CF S ).P(CF 8 ) 2 can be prepared 
by the condensation of trifluoromethylphosphine with chloro- or iodo- 
bistrifluoromethylphosphine in the presence of trimethylamine. 181 

d. Basic Hydrolysis of Trifluoromethyl Derivatives of Phosphorus. Basic 
hydrolysis of most trifluoromethyl derivatives of phosphorus yields fluoro- 
form quantitatively, 174 ' 182 but those containing P — P bonds give a mixture 
of fluoroform and fluoride ions, 174 ' 179 e.g., 

,-™ , ■„ NaOHaq., 20° „„„„ 
(CF 3 ) 3 P ^— >■ 3CHF 3 

(C f 3)2 p C1 """"*.«; 2CHFa 

CF 3 .PI 2 NaOHaq - 2 °°> CHF 3 

.„„.,„„, NaOHaq., 100° «„„„ 

(CF 8 ) 3 P.Ni(CO) 3 ^ > 3CHF 8 

NaOHaa., 20° 
(CF 3 ) 2 P.P(CF 3 ) a — — > 3CHF 3 , F", CO,, 2 -, CF 3 .P acid 

The formation of fluoride ions in the hydrolysis of trifluoromethylpoly- 
phosphines can be explained if the initial step is cleavage of a P— P bond 
followed by decomposition of the products, as exemplified by the mechanism 
that has been established for the hydrolysis of tetrakistrifluoromethyl- 
diphosphine : 

(CF 3 ) 2 P.P(CF 3 ) 2 Na ° Haq > (CF 3 ) 2 PH + (CF 3 ) 2 P.OH 
NaOH aq. NaOH aq. 

CHF 3 + F- + CO s 2 - 2CHF 3 
154 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Reaction of bistrifluoromethylphosphine with basic nucleophiles is believed 
to proceed via formation of the highly-reactive phospha-alkene CF 3 .P : CF, 

e.g., 183 

(CF 3 ) 2 PH "*O^0H*,H (CF 8 ) 2 P-Zl; C F 8 .P:CF 2 ^5^U CF 3 .P(OCH 8 ).CHF 2 

e. Phosphorus Acids Containing Trifluoromethyl Groups. Extensive studies 
have been made of trifluoromethyl oxyacids of phosphorus. Due to the 
powerful inductive effect of the trifluoromethyl group these acids are all 
stronger than their alkyl counterparts, and, as would be expected, trifluoro- 
methylphosphonic acid v CF 3 PO(OH) 2 , is more acidic than trichloromethyl- 
phosphonic acid (see Table 4.6). 

Table 4.6. pK„ Values for Some Acids of Phosphorus 

Formula Name 



V K % 


P-K* 


1-16 


3-93 


1-63 


4-81 


2-38 


7-74 


1-0 


— 


3-08 


— 


101 


— 


1-41 


6-7 


1-97 


6-82 



CF 8 .PO(OH) 2 Trifluoromethylphosphonic acid 

CCl 3 .PO(OH) 2 Triehloromethylphosphonic acid 

CH 3 .PO(OH) 2 Methylphosphonic acid 

(CF 3 ) 2 PO.OH Bistrifluoromethylphosphinic acid 

(CH 3 ) 2 PO.OH Dimethylphosphinic acid 

CF 3 .PHO(OH) Trifluoromethylphosphonous acid 

HjPOg Phosphorous acid 

H 3 P0 4 Phosphoric acid 



Anhydrous trifluoromethylphosphonous acid, which can be prepared by 
allowing a mixture of dibromotrifluoromethylphosphine and methanol to 
warm up from - 196° to 25 °, 184 

CF 3 .PBr 2 +2CH 3 OH — > CF 3 .P(OH) 2 + 2CH 3 Br 

appears to exist almost wholly as the phosphinic acid dimer (XXIII). 
The dimer is a slightly volatile, colourless liquid which dissociates when 

H \^° H-O x/ H 

P P 

FjC/^O— H O^^CFj 

(xxni) 

heated into a monomer which is thought to be mostly of structure 
CF 3 .PH(:0)(OH) rather than CF s .P(0H) g . H owever, the phosphonous 

esters CF 3 .P(OCH 3 ) S! and CF 3 .P.O.CH 2 .CH 2 .0, which do not rearrange 
spontaneously to the phosphinic ester forms, can be prepared by the action 
of methanol and ethylene glycol respectively on dichlorotrifluoromethyl- 
phosphine. Aqueous solutions of trifluoromethylphosphonous acid can be 



n* 



155 



Perfluoroalkyl Derivatives of the Elements 

prepared by the action of water on the compounds CF 3 .PX 2 and (CF 3 ) 2 PX 
(X=ClorI), 18S e.g., 

H.O, 20° 
CF^PClj — — > CF 3 .PHO(OH) 

H.O, 20° H,0, 20° 
(CF 3 ) 2 PI — — > (CF 3 ) 2 P.OH — — >- CF 3 .PHO(OH) + CHF 3 

and by careful alkaline hydrolysis of tristrifluoromethylphosphine followed 
by acidification of the solution with sulphuric acid; 186 the free acid can- 
not be isolated from such solutions since it co-distils with water, but it 
can be obtained as its stable sodium salt, which is a phosphinate, 
CF 3 .PH(:0)(ONa). Trifluoromethylphosphonous acid is quantitatively 
hydrolysed to fluoroform and phosphorous acid when its aqueous solution 
is heated to 100°; it has reducing properties, although it is weaker in this 
respect than phosphorous or methylphosphonous acid. 

Oxidation of aqueous solutions of trifluoromethylphosphonous acid with 
hydrogen peroxide yields the white, crystalline trifluoromethylphosphonic 
acid, CF 3 .PO(OH) 2 , m.p. 81-82°, which is dibasic and one of the strongest 
acids of phosphorus known (see Table 4.6) . 18S 

Bistrifluoromethylphosphinous acid, (CF 3 ) 2 P.OH, a liquid with m.p. 
— 21-1°, b.p. 61-4°, can be prepared from iodobistrifluoromethylphosphine 
via tetrakistrifluoromethyldiphosphoxane : 187 

2(CF 3 ) 2 PI + AgC0 3 -^ (CF 3 ) 2 P.O.P(CF 3 ) 2 (79%) + C0 2 + 2AgI, 

(CF 3 ) 2 P.0.P(CF 3 ) 2 + HC1 -^ (CF 3 ) 2 P.OH(92%) + (CF 3 ) 2 PCl 

Its acid strength has not been measured, but with trimethylamine it yields 
the salt [(CH 3 ) 3 NH]+[(CF 3 ) 2 P.O]~ from which the phosphinous acid can 
be liberated by hydrogen chloride. Bistrifluoromethylphosphinous acid is 
unstable in aqueous solution and decomposes rapidly to give fluoroform 
and trifluoromethylphosphonous acid. 

Bistrifluoromethylphosphinic acid, (CF 3 ) 2 PO.OH, is a stable, viscous, 
colourless, steam-volatile liquid, b.p. 182°, which fumes in the air and is 
the strongest known acid of phosphorus (K a > lO^ 1 ). It is conveniently 
prepared by hydrolysis of dichlorotristrifluoromethyl- or trichlorobistri- 
fluoromethyl-phosphorane : 186 

(CF 3 ) 3 PC1 2 H '°' 2 °°> CHF 3 + (CF 3 ) 2 PO. OH aq. 

JAg,0 

(CF 3 ) 2 PO.OAg(95%) conc - H ' s0 ' > (CF 3 ) 2 PO.OH(95%) 
heat 

JAg,0 

(CF 3 ) 2 PC1 3 H '°' 2 °°> (CF 3 ) 2 PO.OH aq. 
156 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth 

Only one mole of fluoroform per mole of acid is liberated by reaction of 
btetrifluoromethylphosphinic acid with an excess of aqueous sodium 
hydroxide, since the alkali-stable trifluoromethylphosphonic acid is pro- 
duced: 

(CF 3 ) 2 PO.ONa ya0Ha<;l > CF 3 .PO(ONa) 2 + CHF, 

Since the acids CF 3 .PO(OH) 8 and (CF 3 ) 2 PO.OH are highly ionized in 
aqueous solution, their strengths cannot easily be compared with those 
of other strong acids. The conductivities of a whole series of acids in an- 
hydrous acetic acid have been measured in order to make such comparisons 
possible j 186 the order of acid strengths found is 

HC10 4 > (CF 3 ) 2 PO.OH > HBr > HjSO, > CF 8 .PO(OH) 2 , 
• HC1 > (CF s ) 2 AsO.OH > CF 8 .AsO(OH) 2 > CF 3 .CO a H, HN0 3 , n-C 3 F 7 .C0 2 H 

and the relative strengths are listed in Table 4.7, these being based on the 
values of equivalent conductivities at a particular concentration. It is 
obvious that bistrifluoromethylphosphinic acid is one of the strongest acids 
known. 

Table 4.7. Relative Strengths of Acids 



HC10 4 


360 


CF 3 .PO(OH) 2 l 
HC1 J 


9 


0F 3 .CO 2 H ^ 


(CF 8 ) 2 PO.OH 


250 


HNO s \ 1 


HBr 


180 


(CF 3 ) 2 AsO.OH 


3-5 


n-C 3 F,.C0 2 Hj 


H 2 SO« 


32 


CF 3 .AsO(OH) 2 


2-5 





C. Arsenic, Antimony, and Bismuth 

1. Synthesis. Of the perfluoroalkyl derivatives of Group V B elements, 
only some of those of nitrogen can be prepared by fluorination of their 
hydrocarbon counterparts or other nitrogen- containing hydrocarbon deriva- 
tives ; thus perfluoroalkyl derivatives of arsenic, antimony, and bismuth, 
like those of phosphorus, are prepared by indirect methods. 

Arsenic undergoes a simjlar reaction to phosphorus when heated in an 
autoclave with trifluoroiodomethane and yields a mixture of tristrifluoro- 
methylarsine, iodobistrifluoromethylarsine, di-iodotrifluoromethylarsine, 
and arsenic tri-iodide : 1B1 

220—240° 
CF 3 I+ As > (CF 3 ) 3 As, (CF 3 ) 2 AsI, CF 3 .AsI 2 , Asl 3 

When pentafluoroiodoethane is used in place of its lower homologue, tris- 
pentafluoroethylarsine and iodobispentafluoroethylarsine are obtained; 192 
however, although heptafluoropropyl derivatives of arsenic may be present 
in small amounts in the fluorocarbon mixture formed when arsenic is heated 
with heptafluoro-1-iodopropane, attempts to isolate them have failed. 167 

157 



Perfluoroalkyl Derivatives of the Elements 

Trifluoromethyl derivatives of antimony are prepared from the element 
and trifluoroiodomethane with distinctly greater difficulty than are the 
corresponding phosphorus and arsenic compounds, since they are less stable 
thermally. The main product (90%) from the reaction between finely- 
divided antimony and trifluoroiodomethane at 165-175° at pressures 
>50atm is tristrifluoromethylstibine, (CF 3 ) 3 Sb, together with small 
amounts of iodobistrifluoromethylstibine, (CF 3 ) 2 SbI, and di-iodotrifluoro- 
methylstibine, CF s .SbI 2 . 193 

Unsuccessful attempts have been made to prepare tristrifluoromethyl- 
bismuthine by heating bismuth with trifluoroiodomethane and by pyrolysis 
of bismuth tris(trifluoroacetate). 191 Mixed alkyl-perfluoroalkyl bismuth 
compounds have been obtained by heating trimethyl- or triethyl-bismuth 
with perfluoroalkyl iodides, 194 e.g., 

100° 

R F I + R 3 Bi — >■ R F BiR 2 (~85%) + (R F ) a BiR(~15%) 

(R F = CF 3 , n-C 3 F 7 ; R = CH 3 ) 

This method has also been applied to the preparation of mixed methyl- 
trifluoromethyl derivatives of phosphorus (see p. 148), arsenic, and anti- 
mony, 16 * e.g., 

20° 

CF 3 I+2(CH 3 ) 3 Sb » CF 3 .Sb(CH 3 ) 2 (50%) + [(CH 3 ) 4 Sb]+I- 

2. Properties and Reactions of the Trifluoromethyl Derivatives. The boiling 
points of some trifluoromethyl derivatives of arsenic, antimony, and bis- 
muth are listed in Table 4.5 (p. 150), together with those of the corre- 
sponding phosphorus and, if known, nitrogen compounds. 

The reactions and chemical properties of trifluoromethyl derivatives of 
arsenic very closely parallel those of the corresponding phosphorus com- 
pounds, with some differences which can be related to the relative positions 
of arsenic and phosphorus in the Periodic Table. Like their phosphorus 
analogues, the iodotrifluoromethylarsines have played a major role in the 
development of the chemistry of trifluoromethyl arsenic compounds (see 
Fig. 4.4). The halogenotrifluoromethylstibines, although they undergo many 
of the reactions of the corresponding phosphines and arsines, are difficult 
to manipulate since they readily disproportionate to yield tristrifluoro- 
methylstibine and antimony halides as the major products. 

All trifluoromethyl derivatives of arsenic yield fluoroform quantitatively, 
or a mixture of fluoroform and fluoride ions [CF s .AsH 2 , (CF 3 ) 2 AsH, and 
(CF 3 ) 2 As.As(CF 3 ) 2 fall into this category], on treatment with aqueous 
sodium hydroxide ; cyanobistrifluoromethylarsine and dimethylaminobistri- 
fluoromethylarsine are exceptional because they are hydrolysed by water 
alone with the liberation of fluoroform. 196, 19e Since di-iodotrifluoromethyl- 
arsine and iodobistrifluoromethylarsine are stable to water (c/. their phos- 
phorus analogues, p. 156) and react with aqueous sodium hydroxide at 
room temperature to yield fluoroform quantitatively, they cannot be con- 

158 



Nitrogen, Phosphorus, Arsenic, Antimony, and Bi^mvth 



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159 



Perfluoroalkyl Derivatives of the Elements 

verted into the corresponding acids CF 3 .As(OH) 2 and (CF 3 )gAs.OH by 
simple hydrolysis. Oxidative hydrolysis of the iodo-compounds yields tri- 
fluoromethylarsonic acid, CF 3 .AsO(OH) 2 , and bistrifluoromethylarsinic 
acid, (CF 3 ) 2 AsO.OH, M7 which are the only known trifluoromethyl-sub- 
stituted acids of arsenic, although the mercury and silver salts of the un- 
stable bistrifluoromethylarsinous acid have been prepared : 19B 

<CF 3 ) 2 AsI m ° i8 2 t t g ' O > (CF 3 ) 2 As.OAg -^2^% CHF 3 

Trifluoromethylarsonic acid and bistrifluoromethylarsinic acid are con- 
siderably stronger acids than their methyl analogues, and are highly 
ionized in water; however, they are appreciably weaker acids than their 
phosphorus analogues (see Table 4.7, p. 157). 

like most trifluoromethyl derivatives of phosphorus and arsenic, the 
trifluoromethyl derivatives of tervalent antimony and bismuth are readily 
hydrolysed with aqueous sodium hydroxide, their trifluoromethyl groups 
being liberated as fluoroform quantitatively; and it should be noted that 
this property of the trifluoromethyl derivatives of all these Group V 
elements has been of great analytical value since fluoroform is a gas, b.p. 
— 84°, and so can be easily estimated. Detailed investigation of the aqueous 
alkaline hydrolysis of the compounds (CH 3 ) 3 M, CF 3 .M(CH 3 ) 2 , fCF 3 ) 2 M.CH 3 , 
and (CF 3 ) 3 M (M = P, As, or Sb) 168 shows that: (i) the trimethyl compounds 
resist attack by aqueous alkali; (ii) all the trifluoromethyl derivatives yield 
fluoroform quantitatively on treatment with 20 % aqueous sodium hydro- 
xide at 20° ; (iii) for a given central atom M the observed order of increasing 
ease of hydrolysis is 

(CH 3 ) 3 M < CF 3 .M(CH 3 ) a < (CF 3 ) a M.CH 3 < (CF 3 ) 3 M; 

(iv) for a given number of trifluoromethyl groups in the compound the 
rate of hydrolysis increases in the order P < As < Sb. These observations 
are consistent with a hydrolysis mechanism involving initial nucleophilic 
attack by hydroxide ion on the central atom M; hydrolysis is facilitated by 
an increase in the number of highly electronegative trifluoromethyl groups 
attached to M and/or an increase in the electropositive character of M. 
Conversely, Lewis base character is reduced. Thus neither tristrifluoro- 
methylphosphine nor methylbistrifluoromethylphosphine form stable com- 
plexes with boron trifluoride, and the former phosphine does not coordi- 
nate with platinous chloride; in contrast, the following compounds have 
been isolated:™ (CH 3 ) 3 P,BF 3 ; (CF 3 )(CH 3 ) 2 P,BF 3 ; [(CH 3 ) 3 P] g PtCl 2 ; 
[(CF 3 )(CH 3 ) 2 P] 2 PtCl 2 ; and [(CF 3 ) 2 (CH 3 )P] 2 PtCl 2 . Basicity decreases in 
the order P > As > Sb (trifluoromethyl derivatives of As and Sb are very 
weak bases) as shown, for example, by the fact that dimethyltrifluoro- 
methylphosphine reacts with methyl iodide to yield a quaternary salt 
[CF 3 .P + (CH 3 ) 3 I _ ] more readily than does dime thy ltrifluoromethylar sine. 

160 



Oxygen, Sulphur, and Selenium 

Lastly, tristrifluoromethylantimonic acid, H[(CF 3 ) 3 Sb(OH)3], can be ob- 
tained by hydrolysis of tristrifluoromethylantimony dichloride, 198 which is 
a labile liquid formed by the action of chlorine on tristrinnoromethylstibine 
at — 50 . 193 TristrMuoromethylantimonic acid is unique among the anti- 
monic acids, since it is a strong acid (pK — 1-85), and stable in aqueous 
solution. 



VI. OXYGEN, SULPHUR, AND SELENIUM 
A. Oxygen 

1. Perfluoro-ethers. In general perfluoroalkyl ethers are best prepared by 
electrochemical fluorination (see p. 11) of their hydrocarbon counterparts 
or appropriate glycol ethers, 201 e.g., 

, n -,-, > ^ electrochemical , 
(n - CaH ' ) ^° fluorination > ^ C ^^° 

n-C 1 H 9 .O.CH 2 .CH 2 .OH ° leet ™* emica ' 
4 ■ 2 2 fluorination 

n-C 4 F 9 .O.CF 2 .CF s (33% yield of crude product) 
Cyclic perfluoro-ethers can be obtained in analogous fashion, 202 e.g., 

H2C CH2 F 2 C CF 2 

I I electrochemical I I /._„/\ 

I I — — — r. — *■ I I (42%) 

H 2 C^ /CH 2 formation Fj0 ^ /CF2 

Perfluoro-oxetanes can be prepared by photo-initiated cyclo-addition of 
perfluoroacyl fluorides or perfluoroketones to perfluoro-olefins, 208 e.g., 

(CF 3 ) 2 C=0 n , .... (CF,,) 2 C O 

U.T. light (!,„,» 

+ „ „ * — V (50%) 

liquid phase I > I x ' 

CF 3 .CF=CF 2 CF 3 .CF— CF 2 

n-C 3 F,.CF=0 ,. . t n-CsFj-CF— O 

u.v. light 1 i , 

+ ,. ,. . > (73%) 

liquid phase I I v ' 

CF 3 .OF=CF 2 CF 3 .CF— CF 2 

and the structures of the products can be predicted correctly if it is assumed 
that diradical intermediates are involved (c/. p. 49). 

Perfluoroalkyl ethers are considerably more volatile than their hydro- 
carbon analogues, and the boiling point of a perfluoroalkyl ether R F .O.Rj 
lies close to that of the fluorocarbon R F .Ry (e.g., cf. C 2 H 5 .O.C 2 H 5 , b.p. 
34-5°, C a F s .O.C 2 F 5 , b.p. 1° and n-C 4 F 10 , b.p. -1°). Perfluoroalkyl ethers, 
like saturated fluorocarbons, are characterized by high chemical and ther- 
mal stability. They are unaffected by hydriodic acid, do not form addition 
compounds with boron trifluoride, and in fact show few properties usually 

161 



Perfluoroalkyl Derivatives of the Elements 

associated with ethers. Cleavage of perfluoroalkyl ethers can be effected 
with aluminium chloride at elevated temperatures, and this degradation 
provides a means for structure determination, 204 e.g., 

C 3 F 7 .CF 2 .O.CF 2 .C 3 F 7 ^V^> C 3 F,.COCI(30%)+C 3 F 7 .CC1 3 (30%) 

Cyclic perfluoro-ethers containing no perfluoroalkyl substituents in exposi- 
tions undergo ring- opening when heated with aluminium trichloride to 
yield perfluoro-(<w,eo,«-trichloroalkanoyl) chlorides, 205 e.g., 

F 2 F 2 F 2 

/C— C x /C— CC1 3 

/ \ AICI3, 180° / 

FjjC O — : — *■ F 2 C (54%) 

\ / autoclave \ 

X C— (T X C— COC1 

F 2 F 2 F 2 

whereas when an a -perfluoroalkyl group is present, the a-fluorine atoms 
are replaced by chlorine and the ether link remains intact, 206 e.g., 

A1C1», 200° 

n-C 4 F 9 .CF.CF 2 .CF 2 .CF 2 — — — v n-CiF 9 .CCl.CF 2 .CF 2 .CCl 2 (54%) 
I I autoclave I I 

I CH 1 I O ' 

(XXIV) 

Hydrolysis with hot fumi ng sulphuric acid of the trichloro-ether (XXIV) 

1 1 

yields the lactone n-C 4 F 9 .CCl.CF 2 .CF 2 .CO.O, which on aqueous hydrolysis 
gives the keto-acid n-C 4 F 9 .CO.CF 2 .CF 2 .C0 2 H. 

In passing, it may be noted that a CF 2 group in the a-position in a fluoro- 
ether is readily hydrolysed to a carbonyl group, e.g., 



H 2 



> CH 2 C1.C0 2 C 2 H 5 



but the presence of fluorine or perfluoroalkyl substituents on the /3-carbon 
atom considerably reduces the ease of hydrolysis ; thus the ether CHF 2 . 
CF 2 .O.C a H 5 requires treatment with concentrated sulphuric acid to 
hydrolyse it to ethyl difluoroacetate, and perfluoro-ethers show a hydrolytic 
stability comparable with saturated fluorocarbons. 

2. Fluorocarbon Epoxides. Compared with the perfluoro-ethers discussed 
above fluorocarbon epoxides (perfluoro-oxiranes) are highly reactive com- 
pounds and merit attention as a separate class. Details of methods of syn- 
thesis and of reactions have been published only fairly recently and mainly 
in the patent literature. 

Tetrafluoroethylene reacts slowly with molecular oxygen in the dark at 
low temperatures to give mainly a rubbery polymeric peroxide, 
[ — CF 2 .CF 2 .0.0 — ]„, which explodes violently when heated or struck and 

162 



Oxygen, Sulphur, and Selenium 

yields mostly carbonyl fluoride . a07, 808 However, when mixtures of tetra- 
fluoroethylene and oxygen are heated 209 or irradiated with u.v. light, 
y-rays, or X-rays in the presence or absence of free-radical initiators, 810 
tetrafluoroethylene oxide can be obtained, e.g., 

CF 2 :CF 2 °" 100 ° jl3atm - > CF 2 -CF 2 (ca.80%) 

22 CF a CI.CFCl, (Inert -olvent) 2 2V ' 

Other perfluoro-olefin oxides can be prepared similarly, but for these com- 
pounds the preferred laboratory method is reaction of the parent olefins 
with aqueous alkaline hydrogen peroxide, e.g., 211-213 

Perfluorocycloalkenes can also be epoxidized with this reagent, e.g., 214 

F 2 F 2 

F.0^ X CF „„„, „ „ „„„ . _ , F.C^ X CF 



30% H a O^, KOH in methanol^ * | |\ 

co. -20° 



> | \}0 (52%) 



F 2 F 2 

The boiling point of a fluorocarbon epoxide, which is much more volatile 
than its hydrocarbon analogue, lies close to that of its parent olefin, as 
illustrated by the following data : 



I 1 

0F 2 .CF 2 .O, b.p. -64°; CF 2 :CF 2 , b.p. -76° 



CF 3 .CF.CF 2 .O s b.p. -28°; CF 3 .CF:CF 2 , b.p. -29° 



I 1 

(CF a ) 2 C.CF 2 .0, b.p. 4° ; (CF a ) 2 C:CF 2 , b.p. 7° 

F 2 C CF F 2 C CF 

| \)Q, b.p. 26-5°; | ij , b.p. 23-5° 

F 2 C^ c /CF F 2 C^ c/ CF 

F 2 F 2 

This leads to purification difficulties, since epoxides, prepared as above, are 
usually contaminated with parent perfluoro-olefins that often can only be 
completely removed by gas-liquid chromatography or by a chemical 
method. 

163 



Perfluoroalkyl Derivatives of the Elements 

Like their hydrocarbon counterparts, fluorocarbon epoxides undergo 
ring-opening reactions with nucleophilic reagents, but attack occurs at the 
most hindered positions in unsymmetrical compounds : 

Nu" ^-*>C CF 2 y B F Rp0(Nu).CFJ-O- ► B F B F C(Nu).COF ► 

" F" + R F R^C(Nu).CO.Nu 

(N*u = nucleophile ; B p = perfluoroalkyl ; R F = F or perfluoroalkyl) 

If Rp = F, and the group CF.Nu is able to lose hydrogen fluoride [e.g., 
CF.NH.C a H 5 - C:N.C 2 H 5 (see p. 124); CF.OH - C:0 (see p. 165)], the 
final product in the scheme shown above becomes further modified accord- 
ingly- Perfluoropropene oxide, for example, reacts readily with water, 
alcohols, ammonia, and amines to yield derivatives of perfluoropyruvic 
acid: 212 

HjO, dioxan 

->• CF 3 .C(OH) 2 .C0 2 H(68%) 



20" 



CH 3 OH 
/<\ / — > CF 3 .CF(OCH 8 ).C0 2 CH 3 (96%) 



CF 3 .CF CF 2 — ( 



NHs, ether 
cg _ 30O > CF 3 .C(NH 2 ) 2 .CO.NH 2 (41%) 

CjHk.NHj, ether 

-> CF 3 .C(:N.C 2 H 6 ). CO. 1^0^(50%) 



ca. 0" 



Fluoride ion attacks perfluoro-olefin oxides, causing rearrangement to 
carbonyl compounds to occur, e.g., 208,214 



' *. yO\ kf,80° <r^ 

F~ TjO- _CF 2 — — : — » CFj.CF— O" — ► F _ +CF y .COF 

z autoclave • 



■3 



EC- -CF F-C ; C— F F 2 C C:0 

^ I <*F,300 % "I I _ F - + *| I 

F 2 C Xc/ CF 2 aUt0ClaVe 4\ C /CF 2 F,C\ C /CF 2 

F 2 F 2 F 2 



164 



Oxygen, Sulphur, and Selenium 
and the same type of isomerization can be effected with triethylamine : axa ' M B 

, CK V f 

RrRrC CF 2 — ► RfRfC-CF-^-v BrBj. C— COF — >-R 3 N+R p R p CF.COF 

.B 3 ii ^^ BsNy 

[R = C 2 H 5 ; R p = CF 3 , R p = F (carried out at —30° in ether, yield 58%); 
Rp = R F = CF g (carried out at 100° in an autoclave, yield 97%)] 

Lewis acids (A1 2 3 and A1C1 3 are particularly efficient) also catalyse such 

isomerizations, but in the case of an epoxide of type R F .CF.CF 2 .0 (R F = per- 
fluoroalkyl) the product is a trifluoromethyl ketone, R F .CO.CF 3 , not an 
acid fluoride, Rj-.CFg.COF; 214 thus hexafluoroacetone can be obtained in 
high yield by passing perfluoropropene oxide through a heated tube con- 
taining alumina (see p. 87). 

At temperatures above 170°, the epoxides of tetrafluoroethylene and 
perfluoropropene act as sources of difluorocarbene, e.g., 216 

/ \ 175» / S \ 

F 2 C:S + CF 2 — CF.CF 3 antocUve >- CF 2 — CF 2 (39%)+CF 3 .COF 

F 2 F 2 

/°\ 18 • /°\ / C \ 

CFC1:CFC1+CF 2 — CF.CF 3 18S °> FC1C CFC1(85%) Zn - ^"^ FC=CF (70 % ) 

55 

and no doubt this facet of the chemistry of fluorocarbon epoxides will 
receive detailed attention in the future. 

Fluorocarbon epoxides undergo anionic polymerization, and this is 
referred to on p. 169. 

3. Perfluoro-alcohols and -alkoxides. Plenty of evidence is available to 
suggest that in the presence of an ionizing medium a compound containing 
a CF 2 .0H group rapidly loses hydrogen fluoride and changes into an acyl 
fluoride; for example, hydrolysis of perfluorobutyrolactone yields per- 
fluorosuccinic acid : M7 



OF, 



CF 2 — C:0 CF 2 .C0 2 H CF 2 .CO a H 

20° I H 8 o I H,0 CFs.COjH 

+ H 2 ► CF 2 -— * CF 2 -±-+ | 

I I HF I _HF CF 2 .CO a H 
CF a — O CF 2 .OH COF 

and tetrafluoroethylene oxide reacts with water to yield oxalic acid : 9m 

/ N 20° 

CF 2 — CF 2 + H 2 »■ HO.CF 2 .CF s .OH >• H0 2 C.C0 2 H 



165 



Perflttoroalkyl Derivatives of the Elements 

Since no preparative method has yet been discovered which circumvents 
this situation, primary perfluoro-alcohols, e.g., perfluoromethanol (CF 3 .OH), 
perfluoroethanol (CF3.CF2.OH), are unknown. 

Reaction of perfluoroketones with an equimolar amount of anhydrous 
hydrogen fluoride at low temperatures is claimed to yield unstable secondary 
perfluoro-alcohols, 218 a but only perfluorocyclobutanol appears to have been 
isolated and fully characterized. 218b The complete synthesis of this alcohol 
from tetrafluoroethylene is effected as follows : 2W 

CF 2 :CF 2 +NaO.CH 3 ^> OTi ,OF.O.OH,<28*) CEa:CF '' 175 °> 

CF 2 -CF-OCH 3 1750 CF 2 -C(OH) 2 ^ heat 
(65%) > >- 



CF 2 — CF 2 CF 2 — CF 2 

CF 2 — 0=0 „„, . , " CF 2 — CF— OH 

I I 090%) - HF( ~oT U > 1 I • ^ % > 

CF 2 — CF 2 CF 2 — CF 2 

Perfluorocyclobutanol reacts vigorously and quantitatively with water to 
liberate hydrogen fluoride and form perfluorocyclobutanone hydrate, and 
combines exothermically with ketene to yield perfluorocyclobutyl acetate. 
In the absence of moisture it is reasonably stable at room temperature, but 
at elevated temperatures it tends to revert to perfluorocyclobutanone and 
hydrogen fluoride; however, it can be distilled (b.p. 57-58°) without serious 
decomposition occurring. Perfluorocyclobutanol and the corresponding and 
similar perfluoro-1-halogenocyclobutanols (XXV) obtained by reaction of 
perfluorocyclobutanone with the other hydrogen halides appear to be the 
only a-halogeno-alcohols which have been isolated as pure compounds. 218 



CF 2 — C=0 CF 2 — C— OH 

I I + HX »• I I (XXV) 

CF 2 — CF 2 CF 2 — CF 2 

(X = F, CI, Br, or I) 

Several tertiary perfluoro-alcohols are known, and, lacking the element 
of structure CF.OH, they are quite stable under normal conditions. The 
early claim 219 that members of this class can be obtained via treatment of 
esters of perfluoroalkanecarboxylic acids with perfluoroalkylmagnesium 
iodides has been questioned recently, 220, 221 but their synthesis can certainly 
be effected from perfluoroketones and tetrafluoroethylene or perfluoro- 
propene, 222 e.g., 

100°/25 p.s.i.g. „ „ „ irm 1 ^_^„+ 96%H a SO« 
diglyme 



(CF 3 ) 2 CO+CF 2 :CF 2 +CsF „„^ > C 2 F 5 .C(CF 3 ) a .O-Cs+ 



C 2 F 5 .C(CF 3 ) 2 .OH(86%) 
166 



Oxygen, Sulphur, and Selenium 

In the above reaction, no evidence was found for attack by the oxy-anion 
(CF 3 ) a CF.O _ on tetrafluoroethylene despite the fact that the olefin was 
admitted to a pressure vessel containing a pre-formed solution of caesium 
perfluoroisopropoxide (discussed later) ; thus, it seems that the mechanism 
is best visualized as follows : 

(CF s ) 2 CF.O-Cs++CF 2 :CF 2 ,=* (CF 3 ) 2 C:0+CF 3 .CF;Cs+ — ► (CF^CCTCs* 

CF 2 .CF 3 

The simplest tertiary perfluoro-alcohol, perfluoro-t-butanol, a colourless 
liquid, b.p. 45° (c/. t-butanol, b.p. 83°), is obtained in salt form when hexa- 
fluoroacetone is heated alone with caesium fluoride in wet diglyme : 22S 

2( CF 3 ) 2 CO + lC S F ^ + trace, of H i0) QF g+ 

150°, autoclave • " " 

The caesium salt, possibly formed thus : 

O OTCs + o\ O Cs + 0" 

II l> ^-^ V II I 

CFs-CCFa + CsF — »• CF 3 .CF— OF? C(CF,) 2 — ► CF S .CF + C(CF 8 ) 3 

liberates perfluoro-t-butanol (34% overall yield) when treated with con- 
centrated sulphuric acid. The alcohol can also be prepared, but much less 
conveniently, by hydrolysis of perfluoro-t-butyl nitrite synthesized from 
perfluoro(nitroso-t-butane) and dinitrogen tetroxide 220 or by a halogen- 
exchange reaction between antimony pentafluoride and the carbinol 
CC1 3 .C(CF 3 ) 2 .0H, which can be made from hexafluoroacetone and tri- 
chloromethyl-lithium. 221 

Owing to the combined powerful inductive effects of the three trifluoro- 
methyl groups, the acid strength of perfluoro-t-butanol in water at 25° 
(p.K; a 5-2, 221 5-4 220 ) approaches that of acetic acid (pK a 4-76) ; thus this 
alcohol is one of the most acidic saturated alcohols and reacts readily with 
ammonia to form a salt. Perfluoropinacol, prepared from hexafluoro- 
acetone : 224 

2(CF 3 ) a CO + (CH 3 ) 2 CH.OH "^ " gM > (CF 3 ) 2 C(OH).C(OH)(CF 3 ) a (63%)+(CH 3 ) 2 CO 

is also highly acidic (j>K a 5-95), and so is the fluorocarbon gem.-diol hexa- 
fluoroacetone hydrate, CF 3 .C(OH) a .CF 3 (pK tt 6-58; see p. 91 ). 224 

Although trifluoromethanol is unknown, its potassium, rubidium, or 
caesium salt can be prepared indirectly from carbonyl fluoride : 225 

20° 

COF 2 +MF : >■ CF 3 .0~M+ 

acetonitrile 

(M = K, Kb, or Cs) 

167 



Perfluoroalkyl Derivatives of the Elements 

These alkali-metal trifluoromethoxides are white crystalline solids that can 
be isolated by evaporation -of the reaction mixtures in vacuo at low temper- 
atures to prevent dissociation to metal fluoride and carbonyl fluoride (b.p. 
— 83°). Once isolated, the salts are thermally stable at room temperature, 
but rapidly liberate carbonyl fluoride when exposed to moisture. They 
revert to metal fluoride and carbonyl fluoride when heated, the observed 
order of increasing ease of decomposition being 

CsO.CF 3 < BbO.CF 3 <KO.CF 3 ; 

about 10% decomposition of caesium trifluoromethoxide occurs during 
70 minutes at 80°. 

Extension of the above reaction to perfluoroalkanoyl fluorides and to 
perfluoroketones has enabled several other alkali-metal perfluoroalkoxides 
to be prepared and isolated as thermally- stable white solids at room tem- 
perature, e.g., 226,227 

20° 

CF 3 .C0F + CsF > C„F..O-Cs+ 

3 acetonitrile 2 5 

(CF 3 ) 2 C:0+CsF 2 °° > (CF 3 ) 2 CF.O"Cs+ 

acetonitrile * * 

The caesium salts are the most thermally stable, and, in general, these 
undergo < 10 % decomposition to starting materials during 10 minutes at 
50°. Not much use has been made yet of perfluoroalkoxides in synthesis, 
for which purpose they can be generated and used in situ ; examples include 
the conversion of carbonyl fluoride into the perfluoroesters trifluoromethyl 
fluoroformate and bistrifluoromethyl carbonate : 228 

CsF 

COF, > FCO.OCF 3 + (CF 3 .0) 2 CO 

2 -50°/5,000atm. 3 v 3 2 

the preparation of allyl perfluoroisopropyl ether : 226 

20° CH.:CH.CH.Br 

(CF 3 ),CO+CsF > (CF 3 ),CF.O-Cs+ ' " > 

v 8 ' 2 diglyme v 3 ' 2 55° 

(CF„) 2 CF.O.CH 2 .CH:CH 2 + CsBr 

and the synthesis of perfluoromethyl perfluorovinyl ether via reaction of 
caesium trifluoromethoxide with perfluoropropene oxide : 229 

/ \ CsE diglyme , , KOH 
COF 2 +CF 3 .CF CF 2 750| antoe tove > CF 3 .O.CF(CF 3 ).COF »- 



X85-215" 

CF 3 .O.CF(CF 3 ).C0 2 K ? *■ CF 3 .O.CF:CF 2 +C0 2 +KF 



168 



Oxygen, Sulphur, and Selenium 

The last reaction can be modified to allow the preparation of polyethers 
from the epoxides of tetrafluoroethylene and perflaoropropene, polymeriza- 
tion being initiated by a perfluoroalkoxide generated in situ from caesium 
fluoride and an acyl fluoride, e.g. , 280 



dlglyrae 



COn 



CF».GOF + C«F - CjFj.CT F<£- -CF S 

Cs + 



£1 x - 

I 

CF» 



C*F,.O.CF(CF,).CF,.CT FC- -CF 2 —>■ etc. — > 

Cs + I 

CF, 

C^Fj.O— f-CF(CF,).CF 2 .0^— CF(CF,).CF,.0-C 8 + Z^U 



CjF,.0-^-CF(CF,).CF, .0-3— CF(CF,).COF 

The reactive — COF end groups can be converted into — H by heating 
the polymers with aqueous alkali (c/. p. 83), wla or into — F by hydrolysis 
followed by a decarboxylase fluorination procedure. 8311 * The latter process 
provides non-flammable polyethers of type C a F s .O-[-CF(CF 8 ) .CF a .O^C a F 6 , 
which have outstanding chemical resistance (e.g., they are inert towards 
boiling cone. H a S0 4 , molten NaOH, F 8 at 200°, and red fuming HNO s , 
90% H 8 2 , or N 8 H 4 at 25°), excellent thermal stability (stable in air at 
350°), good dielectric properties, and excellent lubricating properties; 
known as Krytox* fluorinated oils and greases, polymers of this type are 
being manufactured on a small scale and appear to hold great promise as 
lubricants and hydraulic fluids for use under severe environmental conditions, 
as encountered, for example, in space vehicles or advanced airoraft.* nb 

4. Perfluoroalkyl Hypofluorites [Perfluoro(fluoroxyalkanes)]. Although 
hypofluorous acid and its salts are unknown, a number of covalent com- 
pounds containing the OF group have been synthesised 888 [e.g., OF a , 
OjN.OF, O3CI.OF, FC(0).OF, CF 3 .C(0).OF, CF 3 .OF, (CF 8 ) 3 C.OF, SF 6 .OF] 
and even some with two such groups [e.g., CF a (OF) a , FO.(CF a ) 6 .OF]. First 
reported in the late 1940's, trifluoromethyl hypofluorite (CF 3 .OF, tri- 
fluorofluoroxymethane) was the only alkyl hypofluorite recorded in the 
literature until the mid-1960's when information on some higher homolo- 
gues and related compounds was published. The class name 'hypofluorite' 
is not meant to imply a positive valenoy for fluorine, but to show that the 
compounds contain an — F bond; systematic names are constructed using 
the term 'fluoroxy' for the OF group. Neither alkyl nor aryl hypofluorites 
appear to be known. 

* Trade name, E.I. du Pont de Nemours & Co., U.S.A. 

12 169 



Perflwroalkyl Derivatives of the Elements 

The simplest method by which trifluoromethyl hypofluorite can be 
prepared in high yield is to pass a mixture of fluorine and carbon monoxide 
in the volume ratio of somewhat more than 2 : 1 through a copper tube at 
ca. 350°. Formation of the hypofluorite occurs in two stages: first fluorine 
and carbon monoxide combine exothermically to yield carbonyl fluoride 
(it is arranged that this reaction occurs in a cooled copper mixing chamber), 
and then this primary product and the remaining fluorine react in the hot 
zone. 233 In 1956 attention was drawn to the analogy between the latter 
reaction and the reduction of aldehydes and ketones to alcohols, and to 
the possibility that other examples of such fluorine addition across a car- 
bonyl group might be found. 234 However, a decade then passed before 
details were published of a simple general method for the conversion of 
perfluorocarbonyl-compounds into the corresponding hypofluorites. This 
method, which corresponds to that used to convert perfluoroalkyl cyanides 
into perfluoroalkyl-JV^-difluoroamines (see p. 130), consists of treating a 
perfluoroalkanoyl fluoride or perfluoroketone with fluorine in a Monel 
vessel at — 78° in the presence of caesium fluoride; 236 almost quantitative 
yields of hypofluorites are achieved, and the reactions, which do not proceed 
in the absence of caesium fluoride, probably involve caesium perfluoro- 
alkoxides as intermediates. Some examples of the use of this fluorination 
technique are given below, including its application to carbon dioxide to 
give difluorobis(fluoroxy)methane. 235 

CF 3 .C(0)F+F 2 -^* C 2 F 5 .OF(96%) 

(CF 3 ) 2 C:0+F a -~* (CF 3 ) 2 CF.OF(98%) 

F(0)C.[CF 2 ] 8 .C(0)F+2F g -^L> FO.[CF a ] B .OF(99%) 

— 78 
CtiW 

CO a +2F 2 — ^ CF 2 (OF) 2 (100%) 

Several other reports 2S6 - 240 have appeared recently dealing with the 
preparation of perfluorinated organic hypofluorites by fluorination of 
oxygen- containing substrates. Noteworthy is the preparation of perfluoro- 
t-butyl hypofluorite in > 95% yield by direct low- temperature fluorination 
of perfluoro-t-butyl alcohol; 234 other tertiary perfluoro-hypofluorites, 
(Rj.) 3 C.OF (R r = perfluoroalkyl), should be capable of synthesis in high 
yields in similar fashion. 

Trifluoromethyl hypofluorite is a colourless toxic gas, b.p. — 95°, with a 
smell reminiscent of fluorine or oxygen difluoride. It remains liquid at 
— 215° and its melting point is unknown. Apparently trifluoromethyl 
hypofluorite can be manipulated without fear of explosion despite the fact 

170 



Oxygen, Sulphur, and Selenium 

that it is thermodynamically unstable with respect to carbon tetrachloride 
and oxygen. When the gas is heated to temperatures above 275° in a nickel 
vessel it decomposes reversibly according to the equation 

CF 8 .OF , COFa+Fj 

and study of the equilibrium over the temperature range 367-467° has 
enabled the molar heat of formation of trifluoromethyl hypofluorite from 
the gaseous elements at 25° to be calculated as 481-4 kcal exothermic. 241 
With the aid of this value it has been estimated that the energy of the 
O— F bond in CF 3 .OF is 47 kcal/mole. 

The strong oxidizing power of trifluoromethyl hypofluorite is illustrated 
by the ease with which it liberates iodine from aqueous potassium iodide 
solution and oxygen from aqueous sodium hydroxide solution, 242 according 
to the equations 

CF s .OF + 3I-+H 2 > C0 2 +I 3 -+2F-+2HF 

CFj.OF + eOH- > C0 3 2 -+4F- + |0 2 + 3H 2 

Possibly these reactions occur via nucleophilic attack on the fluorine of 
the 0— F group, e.g., 

CF,.5W^-I — > IF [^-»- I,+ F-] + CF,.0 _ — * F'+COF, 

JH.O 
CO»+2HF 

as may reactions reported recently between trifluoromethyl hypofluorite 
and steroidal olefins at low temperatures in trichlorofluoromethane, 248 e.g., 





Ao° I ^ AcO \ _ AcO CFj.6 T 

~ F O.CF, F 

No reaction occurs when trifluoromethyl hypofluorite is mixed with 
methane, chloroform, or carbon tetrachloride; but when an electric dis- 
charge is passed through the mixtures explosive decomposition reactions 
occur, 244 e.g., 

CF 3 .OF + CH 4 6leCtriC8park > C + CO + 4HF 

171 



Perfluoroalkyl Derivatives of the Elements 

This is not too surprising since when sparked alone trifluoromethyl hypo- 
fluorite decomposes into fluorine and carbonyl fluoride. Spontaneous ex- 
plosions occur after short induction periods when the hypofluorite is mixed 
with ethylene, acetylene, or cyclopropane. However, trifluoromethyl 
2-fluoroethyl ether is formed quietly and quantitatively when ethylene and 
trifluoromethyl hypofluorite, both diluted with nitrogen, are mixed slowly 
while being irradiated with u.v. light : iU 

CFg-OF+CH^CHg n - v - li8ht > CF 3 .O.CH 2 .CH 2 F(100%) 

The elements of CF 3 0-*-F can also be added across the double bond in per- 
fluorocyclopentene : 

CF„.OF+FC=CF CFs.O.FC CF 2 

I | •* | | iUoo%) 

F 2 C - x /CF 2 F 2 C \ / CF 2 

F 2 F 2 

In contrast, interaction of trifluoromethyl hypofluorite with tetrafluoro- 
ethylene leads, depending on the conditions, to either explosive formation 
of carbon tetrafluoride and carbon monoxide or a polymer reminiscent of 
polytetr afluoroethylene . 

The chemistry of other perfluoroalkyl hypofluorites and of perfluoro[bis- 
(fluoroxy)alkanes] follows from that of trifluoromethyl hypofluorite. Thus 
they can act as sources of 'positive' fluorine, and rapidly oxidize iodide to 
iodine, readily liberate oxygen from aqueous alkali, and displace chlorine 
from sodium chloride, e.g., 886 ' 888 

C 2 F 5 .OF + 2KI+H 2 2S ° > CF 8 .C0 2 K+I 2 +KHF 2 + HF 
C 2 F 5 .OF+4NaOH 25 ° > CF 8 .C0 2 Na+$0 2 + 3NaF+2H 2 

25° 

CF,.CF(OF) 2 +4NaCl > CF s .CO 2 Na+201 2 + 3NaF 

They can easily be defluorinated with mercury or ferrocene to give the 
parent perfluorocarbonyl-compounds, 23 * and readily undergo thermal de- 
composition, e.g., 288 

CFj.CFj.OF " °> CF 4 +COF 2 

(CF 3 ) 2 CF.OF — ^-* CF 4 +CF 3 .COF 

(CF 8 ) s C.OF 26 ° > CF 4 + (CF,) 2 CO 
* (very slow) 

172 



Oxygen, Sulphur, and Selenium 

Such decompositions, which can also be initiated by contacting the hypo- 
fluorites with hydrocarbon material, are believed to involve radical chain 
reactions initiated by homolytic cleavage of an — F bond, 286 e.g., 

CF,.CF 2 .OF — ^-> CF,.0F li .O.+F- 

CF,.CF a .O >- CF s -+COF a (c/.pp. 86, 141) 

CF 3 .+CFs.CF 3 .OF >- CF 4 +CF f .CFjj.O — > etc. 

and 

(CF,),C.OF A -> (CF s ),C.O.+F. 

(CF,),C.O. ► CFj. + fCF^CCMc/. jS-scissioninthe 

t-butoxy radical 8 * 6 ) 
CFj.+(CF,),C.OF ► 0F 4 +(CF,),0.O *- etc. 

Thus all perfluoroalkyl hypofluorites should be manipulated with caution 
and in the absence of hydrocarbon material. Bis(fluoroxy)compounds in 
particular deserve great respect since perfluoro-[2,2-bis(fluoroxy)propane], 
obtainable in low yield by direct fluorination of the mono-sodium salt of 
hexafluoroacetone hydrate, readily decomposes explosively, 288 and difluoro- 
bis(fluoroxy)methane is unstable at room temperature. 240 

An addition reaction occurs between trifluoromethyl hypofluorite and 
carbonyl fluoride at elevated temperatures, the product being bistrifluoro- 
methyl peroxide, CF 3 .O.O.CF, j 241 the reaction is best carried out at 275°/ 
100 atm in a nickel autoclave, to give the peroxide in 93% yield. 24 * A 60% 
yield of the peroxide is obtained when a mixture of fluorine and carbon 
monoxide in the volume ratio of 3:2 is heated to 180° in the presence of 
silver difluoride. 241 Bistrifluoromethyl peroxide, a colourless gas, b.p. — 37°, 
was first prepared in 1933 by Swarts, 2 * 7 who obtained it in low yield by 
electrolysing aqueous solutions containing trifluoroacetate ion; it does not 
react with water or aqueous sodium hydroxide at 25°, but liberates iodine 
quantitatively from aqueous potassium iodide in the presence of u.v. light. 
Bistrifluoromethyl trioxide, CF 3 .O.O.O.CF 3 , is also known. 23 *- 248 A colour- 
less gas, b.p. — 16°, it is best prepared by reaction of oxygen difluoride 
with carbonyl fluoride in the presence of caesium fluoride 248 — a reaction 
which parallels that of trifluoromethyl hypofluorite with carbonyl fluoride 
and which seems capable of extension to perfluoroalkanoyl fluorides and 
perfluoroketones to yield a whole series of bis(perfluoroalkyl) trioxides. 
Bistrifluoromethyl trioxide decomposes very slowly at 25° (half-life 
66 weeks 239 ) to give primarily oxygen and bistrifluoromethyl peroxide, and 
it oxidizes iodide to iodine. 

Trifluoromethyl hypofluorite reacts with sulphur dioxide at 180° to 
yield the compounds CF s .O.SOgF, CF 3 .O.S0 2 .O.CF 3 , CF 3 .O.S0 2 .O.S0 2 F, 

173 



Perfltioroalkyl Derivatives of the Elements 

and CFa.O.SOjs.O.SOjj.O.CFg, while trifluoromethylperoxyfluorosulphonate, 
CF 3 .O.O.S0 2 F, can be obtained from the hypochlorite and sulphur trioxide 
at 245-260°. M3a 

B. Sulphur 

A fair amount of effort has been spent on the chemistry of polyfiuorinated 
organic sulphur compounds during the past two decades, with the result 
that much information on the subject is available. 249 The present brief 
account is concerned mainly with trifluoromethyl derivatives of sulphur 
and, for convenience, is divided into three sections: (1) compounds con- 
taining bivalent sulphur, (2) perfluoroalkanesulphonic acids, and (3) per- 
fluoroalkyl derivatives of sulphur tetra- and hexa-fluoride. 

1. Compounds Containing Bivalent Sulphur. Bistrifluoromethyl disulphide, 
which has played a major role in the development of the chemistry of tri- 
fluoromethyl derivatives of sulphur, can be prepared in high yield by 
thermal reaction of sulphur with trifluoroiodomethane : 250 ' 2B1 

310° 
S+CF 3 I > CF 3 .S 2 .CF 3 (75%)+CF 3 .S 3 .CF 3 (12%)+CF 3 .S 4 .CF 3 (1%) 

or, more conveniently, by fluorination of carbon disulphide with iodine 
penfafluoride : 2B1 

195° 
CS 2 +IF 6 * CF 3 .S 2 .CF 3 (76%)+CF 3 .S 3 .CF 3 (7%) 

The most convenient and cheapest method of preparation, however, appears 
to be the fluorination of thiocarbonyl chloride or trichloromethanesulphenyl 
chloride with sodium fluoride at elevated temperatures in the presence of 
an organic solvent of high dielectric constant, 252 e.g., 

CSCl 2+ NaF "~»^ ■***»», ^^tfVV+CB, 

Reaction of sulphur with a perfluoroalkyl iodide provides a general method 
for the preparation of perfluoroalkyl sulphides, e.g., 4,253 

243° 

S+(CF 3 ) 2 CFI * 



[(CF 3 ) 2 CF] a S(ll %) +[(CF 3 ) 2 CF] 2 S 2 (34%) +[(CF 3 ) 2 CF] 2 S 3 (18% ) 
S+n-C„F 7 I -^> (n-C,F,) s S(ll%)+(u-0,F 7 ) 1 S,(CS%)+(n.C,F 7 ) a 8 J (15%) 

Although bistrifluoromethyl sulphide is not obtained as a product of the 
reaction between sulphur and trifluoroiodomethane, it is readily prepared 
by photochemical decomposition of the disulphide : 25 ° 



CF 3 .S.S.CF 3 "• y " " ght > CF 3 .S.CF 3 (66%) + S 



174 



Oxygen, Sulphur, and Selenium 

Bistrifluoromethyl sulphide, b.p. —22°, resembles a perfluoro-ether in 
its stability to heat and to attack by strong acids and bases. In sharp 
contrast, bistrifluoromethyl disulphide, b.p. 35°, is decomposed rapidly 
and quantitatively at room temperature by dilute aqueous sodium hydrox- 
ide solution with formation of fluoride, sulphide, and carbonate ions but 
no fluoroform. Complete breakdown of two trifluoromethyl groups in this 
manner probably occurs by initial hydrolytic cleavage of the S — S bond 
followed by destruction of trifluoromethanethiol and trifluoromethane- 
sulphenic acid thus formed: 

CF,.S.S.CF S H '°' ° H "> CF 3 .SH + CF 3 .S.X)H 



OH- 



OH- 



F-, S 2 ", CO s 2 - F-, S 2 ", CO, 2 " 

Although trifluoromethanol is unknown (see p. 165) its sulphur analogue 
can be prepared as follows: 261 

CF S .S 2 .CF,+Hg n ' Y ' "^ > (CF 3 .S) 2 Hg(90%) HC '' 2 °°> CF,.SH(99%) 

Perfluoro-n-propanethiol can be prepared by an analogous route. 284 Tri- 
fluoromethanethiol is a gas, b.p. — 37 °, which can be stored and manipulated 
unchanged in a dry Pyrex vacuum system, but which is rapidly and com- 
pletely decomposed by aqueous base to give fluoride, sulphide, and car- 
bonate ions. In the presence of an acceptor for hydrogen fluoride, or in 
ionizing solvents, the thiol decomposes to thiocarbonyl fluoride, which 
often undergoes further reactions. 264 Thus water reacts slowly with tri- 
fluoromethanethiol and carbonyl sulphide is the end-product : 

CF 3 .SH -5i£* HF+CSF a H, ° » COS+2HF 

Anhydrous ammonia similarly dehydrofluorinates the thiol, and the re- 
sultant thiocarbonyl fluoride reacts with unchanged starting material to 
give trifluoromethyl fluorodithioformate (XXVI) and bistrifluoromethyl 
trithiocarbonate (XXVII) : 

OF 3 .SH -Jg. CSF, "^"S CF,.S.CSF "^"S (CF 3 .S) 2 CS 

(XXVI) (XXVII) 

Both (XXVI) and (XXVII) can be prepared solely from thiocarbonyl 
fluoride: 281 * 

F 2 C:S > CF 8 .S- ■ ' " > F-+CF 3 .S.C(:S)F ■ ' > (OF 3 .S) a C:S+F- 

(XXVI) (XXVII) 

175 



PerfluoroaUcyl Derivatives of the Elements 









H 
33 

09 



o 





1*1 




... 






e» 


s? 










p* 


■? 


S" 


1 


CO 


as 


00 


02 






ft. 




o 


o 


o 


o 



s 
op 



(J 
to 

el 

w 

o 
8 
oi 



o 

op 

« 

w 

o 
« 

w 

o 



ft 



m 





ft 




O 


Ift 


CO 


lO 




»-4 


tj 




► o 


a 


op 


o 




M 


&r 




ft ft 



o 



ft 
o 

OS 

o 

6" 



ft 
o 






ft 



ft 
o 



ft 
o 

07 

In 






176 



Oxygen, Sulphur, and Selenium 

Bis(trifluoromethylthio)mercury, (CF 3 .S) 2 Hg, the intermediate in the 
above conversion of bistrifluoromethyl disulphide into trifluoromethanetbiol, 
is a useful reagent for the introduction of the CF 3 .S— group into molecules 
(cf. the role played by [(CP 3 )gN]jHg in the synthesis of (CF 3 ) 2 N— deriva- 
tives [p. 137]) since it reacts as a mild trifluoromethanethiolating agent 
towards compounds containing reactive substituents (see Fig. 4.5). In 
contrast to the high-melting (175°), water-insoluble bis(methylthio)mercury, 
it is a low-melting (39-40°) white, crystalline solid which sublimes readily 
and is soluble in water and many organic solvents. Tests on rats have shown 
that bis(trifluoromethylthio)mercury causes severe damage to the stomach 
and kidneys when taken orally, and it will also cause serious skin burns. 
Although the mercurial can be obtained in excellent yield (90%) by photo- 
lysis of bistrifluoromethyl disulphide in the presence of mercury, a more 
direct synthesis is available: 26s 

250° 

2CS 2 + 3HgF 2 — — > <CF 3 .S) 2 Hg(72%)+2HgS 

autoclave 

Interestingly, trifluoromethylthiosilver, CF 3 .SAg, can be prepared by 
heating carbon disulphide with silver fluoride. 2611 * 

Low-temperature chlorination of bis(trifluoromethylthio)mercury yields 
trifluoromethanesulphenyl chloride, a useful reagent best prepared by 
photochemical chlorination of bistrifluoromethyl disulphide 261 or reaction 
of trichloromethanesulphenyl chloride with sodium fluoride in an aprotic 
solvent. 262 Trifluoromethanesulphenyl chloride is a golden-yellow liquid, 
b.p. —0-7°, which behaves as a typical acid chloride in its reactions with 
ammonia, amines, and, rather surprisingly, phosphine : *•* 

(95%)CF s . Sa .CF s sSiS, /~ J ^ CF,.S.NH 2 (98%) 



CF,.SH,20°\ / -45' 

/ 



CF ' SC1 < ^he^'' CF»-S.NH.C,H 5 (76%) 
<M%)OT,.g,.OT, < H ' S ' 2 °° / \ 95 P °' 20 > (CF,.S),P + (CF a .S) 2 PH 

It also reacts readily with mercury and with substrates containing S — H 
bonds, 261 as shown above, and rapidly attacks silver cyanide, cyanate, 
thiocyanate, or selenocyanate at room temperature to yield the corre- 
sponding trifluoromethanesulphenyl pseudohalides. 283 

Homolytic cleavage of /the S — CI bond in trifluoromethanesulphenyl 
chloride can be effected with the aid of u.v. light, X-rays, or azo-initiators, 
and a study has been made of the free-radical addition reactions that occur 
between the sulphenyl chloride and fluoro-olefins. 264 Aryl trifluoromethyl 

177 



Perfluoroalkyl Derivatives of the Elements 

sulphides can be prepared from trifluoromethanesulphenyl chloride 
e.g. 266 > 266 

CF s .SCl+C 6 H 5 .MgCl — ~ 10to0 ° > C 6 H 6 .S.CF 3 (54%) 
tetrahydrofuran ° 3 

CF,SC1 + C 6 H 8 -±^_> C,H 5 .S.CF 3 (57%) 

25° 

CF S .SC1+C 6 H 5 .0H — -> p-HO.C 6 H 4 .S.CF 3 (73%) 

chloroform-pyiidine ° * 3 

A detailed study has been made of the hydrolysis of trifluoromethane- 
sulphenyl chloride with water. 267 In short, the initial product is trifluoro- 
methanesulphenic acid, which cannot be isolated since it disproportionates 
rapidly to give trifluoromethanethiol and trifluoromethanesulphmic acid: 

2CF 3 .S.OH >- CFj.SH+CFj.SOaH 

The sulphinic acid is stable in aqueous solution and can be isolated as its 
sodium salt, CF 3 .S0 2 Na, H 2 0, which is conveniently prepared from a 
solution of the acid obtained by reduction of trifluoromethanesulphonyl 
chloride with zinc dust and water in the absence of air : 267 

CF„.SC1 C1 ';° 8 °> CF 3 . 80,01(98%) Zn ;°»°> CF 3 .SO a H(100%) 

Trifluoromethanesulphinic acid and its salts are the only trifluoromethyl 
derivatives of sulphur which liberate fluoroform (this occurs quantitatively) 
on treatment with aqueous sodium hydroxide solution at 100°; the other 
derivatives either decompose completely to fluoride, sulphide, and carbonate 
ions (e.g., CF 3 .SH, CF 3 .S 8 .CF 3 ) or undergo no change (e.g., CF 3 .S.CF 3 , 
CF 3 .S0 3 Na, CF 3 .SF 5 ). 

2. Perfluoroalkanesulphonic Acids. Trifluoromethanesulphonic acid was 
the first perfluoro-sulphonic acid to be reported. It is conveniently prepared 
from either carbon disulphide or methanesulphonyl halides : **•• M9 

CS a **^ (CF 8 .S) 2 H g (72%) ■*,--»■*»», C F 3 .SO 8 H(70%) 

\^ electrochemical fluorination / (87%) 

B a( OH),. 8 H,0,H,0 ) (CFs . SO3)2Ba(100%) conc.H.SO,, ^^^ 

The second method has been developed as a general route to perfluoro- 
alkanesulphonic acids. 270 The yield of sulphonyl fluoride in the fluorination 
step decreases steadily as the size of the alkyl group increases ; thus, electro- 

178 



Oxygen, Sulphur, and Selenium 

chemical fluorination of ethanesulphohyl chloride, gives perfluoroethane- 
sulphonyl fluoride in 79% yield, whereas only a 25% yield of perfluoro- 
n-octanesulphonyl fluoride is obtained from n-octanesulphonyl chloride. 
Electrochemical fluorination of arenesulphonyl halides gives low yields of 
the corresponding perfluorocyclohexanesulphonyl fluorides. 271 

Perfluoropropane-2-sulphonyl fluoride is formed when perfluoropropene 
is treated with sulphuryl fluoride and caesium fluoride in diglyme at 100°, 
but under similar conditions tetrafluoroethylene is converted into the 
sulphone (C 2 F 5 ) 2 S0 2 in 83% yield; unsymmetrical perfluorosulphones can 
be prepared from perfluoro-olefins, perfluoroalkanesulphonyl fluorides, and 
caesium fluoride. 299 These reactions can be rationalised on the basis of 
perfluorocarbanion intermediates : 

— — SO IP 

Bp.CF:CF a +F- v - " R P .CF.CF 8 — '-+ F-+Rf.CF(CF s ).S0 2 F 

Kf.T5f.CF, 

R,.eF(CF 3 ).SO 2 .0F(CF 3 ).R^+F- 
(R p , R F = F or perfluoroalkyl) 

Perfluoroalkanesulphonyl fluorides are colourless volatile substances 
(e.g., CF 3 .S0 2 F, b.p. -22°; n-C 6 F 13 .SO a F, b.p. 114°) with an odour re- 
miniscent of a saturated fluorocarbon. They react directly with ammonia 
and aniline to yield the corresponding amides, and are hydrolysed rapidly 
and quantitatively by aqueous solutions of sodium, potassium, and barium 
hydroxide to give the corresponding metal sulphonates. Water hydrolyses 
a perfluoroalkanesulphonyl fluoride only slowly, even at 100°. Long-chain 
alkali-metal perfluoroalkanesulphonates are markedly surface-active, and 
this property has been made use of in industrial applications, e.g., they 
are used to minimize spray in chromium-plating baths since they form 
stable foams even in the presence of hot concentrated acids. A large pro- 
portion of current textile proofing agents are based on derivatives of per- 
fluoroalkanesulphonic acids, and it has been suggested that the high ther- 
mal stability and long liquid ranges of potassium perfluoroalkanesul- 
phonates may lead to their use as high-temperature lubricants and heat- 
exchange fluids. 

Perfluoroalkanesulphonic acids can be liberated from their salts with 
100% sulphuric acid; they are relatively volatile (e.g., CF 3 .S0 3 H, b.p.162"; 
n-C 6 F 13 .S0 3 H, b.p. 238°) hygroscopic (yielding initially solid monohydrates) 
oils or waxy solids. The anhydrous acids are stable at 400° in the absence 
of air, and are unaffected by concentrated nitric acid at 160°. Apart from 
the expected increase in surface activity, the longer-chain perfluoroalkane- 
sulphonic acids closely resemble trifluoromethanesulphonic acid, some 
reactions of which and of its derivatives are shown in Fig. 4.6. 

179 



Perfluoroalkyl Derivatives of the Elements 



O 



o 

to 
P. 



P 

o 

* 

O. 



o 

o 

to 
,' w 

p 



<J^ 


jj6' 


^^ 


8 


o B 


^ 

O^ 




s5< 




PQ 


""» O 


60 




~c 


< 








o 

en 




o 

CO 






n 


U. 




U. 


O 




P 






gs 


ID 

O 


CO, 
O 




o 
o 


o 

so 


e 

O 


SB 
D 




1 

.a 



"8 



a. 



s 






180 



Oxygen, Sulphur, and Selenium 

Trifluoromethanesulphonic acid is a colourless, fuming, oily liquid re- 
sembling sulphuric acid. It chars paper, reacts vigorously with water, and 
on exposure to moist air forms a solid monohydrate that is dehydrated 
only slowly by hot concentrated sulphuric acid. Conductivity measurements 
on solutions of trifluoromethanesulphonic acid in anhydrous acetic have 
revealed that it is a stronger acid than perchloric acid. 278 It liberates hy- 
drogen chloride from sodium chloride, and silver trifluoromethanesul- 
phonate is soluble in benzene, like silver salts of other very strong acids 
(e.g., AgC10 4 ). Besides being responsible for the powerful acidity of tri- 
fluoromethanesulphonic acid, the strong inductive effect of the CF 3 .S0 2 
group causes alkyl trifluoromethanesulphonates (e.g., CF3.SO2-O.CgH5) to 
be excellent alkylating agents, since alkyl-oxygen fission is facilitated; 278 ' 27 * 
it also makes trifluoromethanesulphonic anhydride a good promoter of 
esterification reactions, which offers some advantages over trifluoroacetic 
anhydride (see p. 77) in this connection. 873 Preliminary experiments have 
shown that mixtures of hydrogen peroxide and trifluoromethanesulphonic 
acid or hydrogen peroxide and trifluoromethanesulphonic anhydride, i.e. 
potential sources of peroxytrifluoromethanesulphonic acid, do not have the 
oxidizing power of peroxytrifluoroacetic acid (see p. 78). 27S Electrolysis of 
trifluoromethanesulphonic acid containing sodium trifluoromethanesulpho- 
nate at -23° yields the peroxide CF 3 .SO a .O.O.SO a .CF 3 , an unstable com- 
pound that decomposes explosively at ca. 10° to yield mainly sulphur tri- 
oxide, hexafluoroethane, and trifluoromethyl trifluoromethanesulphonate. 274 

Perfluoro(a-carboxyalkanesulphonic) acids can be prepared by alkaline 
hydrolysis of perfluoro-/J-sultones obtained from terminal perfhioro-olefins 
and freshly -distilled sulphur trioxide, e.g., 277 

80°/3atm. 7* I ,™o/» Na0H »«»•• 20 ° 
CF 2 :CF 2 +S0 3 > J I (93%) > 

o — so 2 

_ cation-exchange resin _ _ 

Na0 4 C.CF 2 .SO s Na " > HOgC.CF2.SO3H 

These /?-sultones react with cold water to yield perfluoro(a-carboxyalkane- 
sulphonyl) fluorides, which lose carbon dioxide when heated with water to 
give the corresponding a-monohydrofluoroalkanesulphonyl fluorides. De- 
hydrofluorination of the last-named compounds yields perfluoroalkene- 
sulphonyl fluorides, e.g., 278 



CF 8 


100° 


CF 3 — CF— CF 2 

so 2 -o 


(85%) 


HjO, heat 
>- 






sealed tube 






[CF s .CF(CO a H).SO s F] -^ 


>• CF t . 


CHF.SOjF(63%) 


KCl/Cr t 3 
509740 mm 












CF,:CF. 80,^(51%) 














181 



Perfluoroalkyl Derivatives of the Elements 

3. Perfluoroalkyl Derivatives ol Sulphur Tetra- and Hexa-fluoride. Per- 
fluoroalkylsulphur pentafluorides, R F .SF 5 , and bisperfluoroalkylsulphur 
tetrafluorides, (R r ) 2 SF 4 , are prepared by oxidative fluorination of com- 
pounds containing carbon-sulphur bonds ; in some instances these products 
are accompanied by small amounts of the much rarer perfluoroalkylsulphur 
trifluorides, R F .SF S . 

Electrochemical fluorination of dialkyl sulphides is the best general 
method of preparation of perfluoroalkyl derivatives of sulphur hexa- 
fluoride : 

B.S.R electrochemica > R p .SF B + (R P ) 2 SF 4 +breakd<mn products (fluorocarbons, SF 6 ) 
fluorination 

The fission of one C — S bond in a dialkyl sulphide accompanied by complete 
fluorination with oxidation of the sulphur atom to its highest valency state 
leads to formation of the corresponding perfluoroalkylsulphur pentafluoride ; 
this reaction usually predominates over complete fluorination of intact 
sulphide molecules, and the bisperfluoroalkylsulphur tetrafluoride is often 
formed in only low yield, 279-281 e.g., 

electrochemical CT gp (67%) + (CT , BF (6%) 
fluorination 

(CA)2S ^fluoril" C 1 F f .SF,(9%)+(C i F I ) i SF 4 <C2%) 

(n-CaH^S *^^> n-C s F 7 .SF 5 (18%) + (n-C 3 F 7 ) 1! SF 1 (13%) 

Details of the electrochemical fluorination of some other sulphur com- 
pounds are given in Table 4.8. It should be noted that isolation of pure 
compounds from the often complex reaction products is usually difficult. 

Besides being a product of the electrochemical fluorination of dimethyl 
sulphide (see above) and of carbon disulphide (see Table 4.8), trifluoro- 
methylsulphur pentafluoride can be prepared in 10-20% yield by fluorina- 
tion of methanethiol with cobalt trifluoride or with fluorine in the presence 
of silver difluoride, and in 40% yield by fluorination of carbon disulphide 
with cobalt trifluoride : 28a 

CS a C ° F - 20a - 250 ; CF S .SF 6 <40%)+CF 4 + SF 6 

Direct fluorination of carbon disulphide gives a complex mixture of products 
which contains trifluoromethylsulphur trifluoride as well as the penta- 
fluoride : Ma 

CF 3 .SF 6 , CF 3 .SF 3 , CF Z (SF 6 ) 2 , SF 5 .CF 2 .SF 3 , CF 4 , CSF 2 , SF 4 , S 2 F 10 , SF 6 
182 



Oxygen, Sulphur, and Selenium 
Table 4.8. Results of the Electrochemical Fluorination of Some Sulphur Compounds 



Solute 


Products (approx. yield %) 


Bef. 


CS 2 


CF 3 .SF 5 (>90%) 
CF 2 (SF 6 ) 2 (0-5%) 
CF 2 (SF 8 ) 2 (0.5%) 


279 


(CH^.S), 


CF 3 .SF 6 (2%) 
CF 2 (SF 5 ) 2 (1%) 
(CF 2 .SF 4 ) 3 (-) 
CF 3 .SF 4 .CF 2 .SF 6 (-) 


280 


CHj.S.tCHjj^S.CH, 


CF 3 .SF 5 (14%) 

CaFs.SFi.CFjtloyo) 

(C 2 F 6 ) 2 SF 4 (1%) 

(C 2 F 4 .SF 4 ) 2 <2%) 

CF 3 .SF 4 .(CF 2 ) 2 .SF 5 (2%) 


280 


(CHjJgSOj 


(CF 3 ) 2 S0 2 (4%) 


287 


(n-C 4 H,),SO 


n-C 4 F B .SF s (2%) 
(n-C 4 F„) 2 SF 4 (2%) 


281 


HOjC.CHj.SH 


CF 3 .SF 6 (3%) 

FOC.CF 2 .SF 5 (2%) 

H0 2 C.CF 2 .8F 3 (3%) 


288 


n-CjHu.SH 


n-C 8 F 17 .SF 5 (18%) 
C 2 F 5 .O.C 2 F 5 (10%) 


287 


1 1 
CH 2 .CH 2 .O.CH 2 .CH g . S 


280 




C 2 F 5 .SF 5 (4%) 






C 2 F 6 .0.(CF 2 ) 2 .SF 6 (5%) 






CF 2 .CF 2 .O.CF 2 .CF 2 .SF 4 (20 % ) 





The best method of preparation of trifluoromethylsulphur trifluoride 
appears to be the vapour-phase fluorination of carbon disulphide with silver 
difluoride: 284 

CS 2 AgF '> CF 3 .SF 3 (28%), CF 3 .SF 5 (8%), SF 4 (37%), SF 6 (2%) 

Trifluoromethylsulphur trifluoride was the only well-established per- 
fluoroalkyl derivative of sulphur tetrafluoride until it was found recently 
that perfluoroisopropylsulphur trifluoride can be obtained, together with bis- 
perfluoroisopropylsulphur difluoride, by the caesium fluoride-catalysed reac- 
tion which occurs between sulphur tetrafluoride and perfluoropropene : a85 

CF S .CF:CF 2 + SF 4 ^ 15 °> (CF 3 ) 2 CF.SF 8 (40%)+[(CF 3 ) 2 CF] 2 SF 2 (15%) 

Similarly, the compound (CP 3 ) a CF.SF 2 .CF 3 can be prepared from per- 
fluoropropene and trifluoromethylsulphur trifluoride at elevated temper- 
atures in the presence of caesium fluoride. It has been suggested that these 

183 



Perfluoroalkyl Derivatives of the Elements 

reactions occur via attack of caesium fluoride on the olefin, to yield the 
perfluoroisopropyl anion which subsequently attacks the sulphur atom of 
sulphur tetrafluoride or of its mono-perfluoroalkyl derivative. 285 

Perfluoroalkyl derivatives of sulphur hexafluoride, which is an extremely 
stable compound, show a not unexpected high degree of chemical and 
thermal stability. Thus trifluoromethylsulphur pentafluoride can be re- 
covered unchanged after treatment with 20% aqueous or ethanolic potas- 
sium hydroxide at 100° for 2 x / g days, 878 and it is a better electrical insulator 
than sulphur hexafluoride. 88 * Thermal decomposition of trifluoromethyl- 
sulphur pentafluoride occurs quantitatively at 400-500° according to the 
equation 

CF 3 .SF 5 > CF 4 +C 2 F e + SF 4 

This result can be explained by any of several free-radical mechanisms, 
e.g., 

CF,.SF 5 400 ~ 50 °°> CF 3 .+F. + SF< 

CF S .+F. >■ CF 4 

2CF, > C 2 F 6 

and this is supported by several pyrolytic reactions which have been carried 
out with trifluoromethylsulphur pentafluoride and other perfluoroalkyl 
derivatives of sulphur hexafluo^ide, ^87,28 * e.g., 

518° 

CF 3 .SF 6 +CF 3 .N:CF 2 >• (CF 3 ) S N + SF 4 

n-C 8 F„.SF 5 +Br a -^£!> n-C 8 F 17 Br+n-C 8 F 18 + SF t 

650° 
(n-O t F,) 2 SF 4 +Br 2 *■ n-C 4 F 9 Br + SF 4 

In contrast to the hexafluoride, sulphur tetrafluoride shows acceptor 
properties towards Lewis bases, and is rapidly hydrolysed by water to 
thionyl fluoride : M0 

SFj+HjO >• SOF a +2HF 

Trifluoromethylsulphur trifluoride and perfluoroisopropylsulphur trifluoride 
also hydrolyse rapidly, and careful treatment of the latter with water and 
ethanol yields the corresponding sulphinic acid and ethyl sulphinate, 
respectively : * 86 

/•^f' (CF 3 ) 2 OF.S0 2 H(18%) 
(CF 3 ) 2 CF.SF,-/ 

\ C,H 8 .OH ) (OFf)$OTj80l0l H 5 (8-7%) 

184 



Oxygen, Sulphur, and Selenium 

Bisperfluoroisopropylsulphur difluoride, however, does not react with 
water even at 100°; it has been suggested that this stability to hydrolysis 
is due to a steric effect, since molecular models reveal that the periphery 
of the molecule is essentially composed of fluorine atoms. 885 Pyrolysis of 
bisperfluoroisopropylsulphur difluoride at 200° in an autoclave yields per- 
fluoropropane-2-sulphenyl fluoride, (CF 3 ) 2 CF.SF, which is possibly the only 
known sulphenyl fluoride. 286 The sulphenic ester (CF 3 ) 2 CF.S.O.CF(CF 3 ) 2 
is formed when the difluoride is heated with boric oxide at 150°. 

Interestingly, the only known alkylsulphur trifluoride is a monofluoro- 
butylsulphur trifluoride that was prepared by the addition of di-n-butyl 
disulphide to a slurry of silver difluoride in 1,1,2-tricblorotrifluoroethane; 
fluorination of di-aryl disulphides in this manner provides a general route 
to arylsulphur trifluorides, 284 e.g., 

,„_.„ AgF 8 lnCF 8 Cl.CTCl^ „,„,;, 

(C.H^Sj ^-^- > C 6 H 5 .SF g (61/o) 

The reactions of monofluorobutylsulphur trifluoride and the arylsulphur 
trifluorides parallel those of sulphur tetrafluoride : they react vigorously 
with water to yield sulphuric acids and will convert compounds containing 
carbonyl and carboxyl groups to the corresponding difluoromethylene and 
trifluoromethyl compounds, respectively, e.g., 

C 6 H B .SF 3 S2l2; C„H 6 .S0 2 H 
C 6 H 5 .CHO+C 6 H 5 .SF 3 ^±^ C 6 H 5 .CHF 2 (80%)+C 6 H 5 .SOF(89%) 

1 20° 

CH 3 .[CH 2 ] 5 .C0 2 H+C 6 H 5 .SF 3 » CH 3 .[CH 2 ] 5 .CF 3 (28%) 

The simplest alkylsulphur pentafluoride known is 2-chloroethylsulphur 
pentafluoride, which is prepared by heating ethylene with sulphur chloride- 
pentafluoride : 

CH,:CH 1 + BI , I Ca 90 ° /2S0atm - > CH 2 Cl.CH a .SF B (47%)+Cl.[CH 2 .CH 2 ] 2 .SF 5 

Several other 2-chloroalkylsulphur pentafluorides have been prepared from 
alkenes and sulphur chloride-pentafluoride;* 90 ' 291 they are stable to aqueous 
alkali but are dehydrochlorinated by ethanolic potassium hydroxide, e.g., 

CH 2 C1.CH 2 .SF 5 K0Haa -' C ; H ' 0H > CH 2 :CH.SF 5 (85%) 
* a neat 

The adduct obtained from trifluoroethylene and sulphur chloride-penta- 
fluoride yields perfluorovinylsulphur pentafluoride when treated with pow- 

185 



13 



Perfluoroalkyl Derivatives of the Elements 



z 


■> 


? 


v> 










C 








»3 


I* 




s 


b 


u 


U 


J 


V 


. 


s 



















*r 


R 










~- 


.A 

91 


p 


c 




t) 




y 
ii 
X 

X 

w 

x 
o 



o 




:» 
-e 



<t 



ft; 
6 

IH 



186 



Transition, Metals 
dered potassium hydroxide: 2 * a 

CF •CHP + SP CI ( c « H »- co )»0». carbon tetrachloride 
"' 5 150" ~~ *" 

CF 2 C1.CHF.SF 5 K0H '" 8i ; e P a ; tr0lenm > CF 2 :CF.SF 5 (82o/ ) 

Arylsulphur pentafluorides can be prepared by fluorination of di-aryl 
disulphides or arylsulphur trifluorides with silver difluoride at 120-130 . 298 

C. Selenium 

Trifluoromethyl and heptafluoro-n-propyl derivatives of selenium are 
known and have been studied in some detail. The chemistry of the tri- 
fluoromethyl compounds is summarized in Fig. 4.7 ; the heptafluoro-n-propyl 
compounds undergo analogous reactions except that so far attempts to 
prepare the selenol n-C 3 F 7 .SeH and the trichloride n-C 3 F r .SeCl 3 have 
failed. 

Bistrifluoromethyl selenide and diselenide are obtained when selenium 
is heated with trifluoroiodomethane in an autoclave : M4 

260—285° 

CF„I + Se >. CF 3 .Se.CF 3 (45-50%)+CF 3 .Se 2 .CF 8 (10-15%) 

They can also be prepared by heating selenium with mercuric 295 or silver 296 
trifluoroacetate, or from selenium dioxide and trifluoroacetic anhydride at 
260-280° j 296 presumably these reactions involve the formation and sub- 
sequent thermal decarboxylation of polyselenium trifluoroacetates. 296 Bis- 
heptafluoro-n-propyl mono- and di-selenide are formed when selenium is 
heated with silver heptafluoro-n-butyrate. 297 Unsuccessful attempts have 
been made to prepare trifluoromethyl derivatives of selenium by the electro- 
chemical fluorination of carbon diselenide and of dimethyl selenide. 279 

The trifluoromethyl derivatives of selenium are characterized by: (i) 
greater reactivity than their sulphur analogues, ascribed to weaker Se— Se 
and C— Se bonds; (ii) an ability to form the trichloride CF 3 .SeCl 3 , which 
has no sulphur analogue ; and (iii) formation of the seleninic acidCF s .SeO.OH 
as the most stable oxy-acid, in contrast to the sulphonic acid CF s .S0 2 .OH. 
These trends are generally in accord with those observed with other organo- 
derivatives of selenium and sulphur. 



VII. TRANSITION METALS 

Much progress has been made in the study of fluorocarbon derivatives 
of transition metals during the past decade 300 and the discussion that 
follows is very much an introductory account. So far interest has centred 
mainly around the ground-work areas of synthesis and structure and bond- 

187 



Perfluoroalkyl Derivatives of the Elements 

ing, and the majority of derivatives prepared have been carbonyl com- 
pounds. 

A. Preparation 

1. From PerfluoroalkanoyI Halides or Perfluoroalkanecarboxylic Acid 
Anhydrides and Alkali-metal Derivatives of Metal Carbonyls. It was briefly 
disclosed in 1959 301 that the first perfluoroalkyl derivative of a transition 
metal, namely trifluoromethylpentacarbonylmanganese, can be prepared 
by a method used earlier to obtain its methyl analogue : 

R.COCl+NaMn(CO) 5 tetrah r drofara > n NaCl+R.CO.Mn(CO) 6 -^-> R.Mn(CO) 5 + CO 

(R = CF„ or CH 3 ) 

Several other perfluoroalkyl derivatives of manganese have since been pre- 
pared in this way, 302, 303 and the method has also been successfully adapted 
to the preparation of perfluoroalkyl derivatives of rhenium, 802 iron, 304 and 
cobalt : ** 

t> nnm , it -.r,/-i^. tetrahydrofuran _ __ _, __ heat (t°) 
R F .COCl+NaM(CO) 5 =^-- >- R F .CO.M(CO) 5 



25° in vacuo 

R P .M(CO) 5 (80-100%) +CO 
(M = Mn;Rj. = CF 3 , C 2 F 5 , n-C 3 F 7 , i-C 3 F 7 ; t" = > 80°. M = Re; R F = C 2 F 6 ,n-C 3 F 7 ; 
t" = 120-150°. LiMn(CO) 5 has also been used in this reaction) 

2R F .COCl+Na 2 Fe(CO) 1 tetrah ^° fUra > (R F ) a Fe(CO) 1 (13-15%)+2CO+2NaCl 
(R F = C 2 F 6 , n-C 3 F 7 ) 

Rp.COCl+LiCofCO)! tet "^ y f JOf '^ a > R F .Co(CO) 4 (upto37%)+CO+LiCI 
(R F = CF 3 , C 2 F 5 , n-C 3 F,) 

In some cases fluorocarbon acid fluorides and anhydrides have been used 
instead of acid chlorides, e.g., 303 - 305 ' 806 

(CF 3 ) 2 CF.COF+LiMn(CO) 5 to***"*™? 
82 l ' 5 -80 to 25° 

(CF 3 ) 2 CF.CO.Mn(CO) 5 (62%) - 11 °' /1 atm > : (CF 3 ) 2 CF.Mn(CO) 5 (90%)+CO 
(CF,CO) 2 +N aCo(CO) 3 P(C 6 H 6 > 3 ^drofura n 

CF 3 .CO.Co(CO) 3 P(C 6 H s ) 8 P '^™> OF 3 .Co(CO) 3 P(C 6 H s ) 3 

Some sources of carbonylmetal anions can also be used to prepare com- 
plexes containing a-bonded unsaturated fluorocarbon groups from per- 

188 



Transition Metals 

fluoro-olefins, presumably via an addition-elimination mechanism (c/. 
p. 29), e.g., 3 " 

78° 

CF 2 :CF 2 +Na+[Ke(CO) 5 ]- > CF 2 :CF.Re(CO) 5 (63%) +NaF 

tetrahydrofuran * 

Similarly, reaction of perfluoropropene with sodium pentacarbonylmangan- 
ate yields the propenyl complex CF 3 .CF:CF.Mn(CO) 6 , a product that is 
also obtained (as the fraws-isomer) when perfluoroallyl chloride is treated 
with alkali-metal pentacarbonylmanganates; 303,307 in the latter type of re- 
action the isomeric allyl complex forms first and rearranges spontaneously, 
possibly by an intermolecular process (c/. p. 193) : 

CF 2 :CF.CF 2 Cl+Mn(CO)- >■ CI" + CF 2 :CF.CF 8 .Mn(CO) 5 >- 



CF s .CF:CF.Mn(CO) 6 

Perfluorovinyl derivatives of nickel, palladium, and platinum have been 
prepared from perfluorovinyl-lithium or perfluorovinylmagnesium brom- 
ide, 308 e.g., 

20° 

<rans-[(C 2 H 6 ) 3 P] 2 NiCl 2 + CF 2 : CF . MgBr 



tetrahydrofuran 

«rans-[(C 2 H 5 ) 3 P] 2 Ni(CF:CF 2 )Br(17 % ) + 
«rans-[(C 2 H 6 ) 3 P] 2 Ni(CF: CF 2 ) 2 (26 % ) 

ci«-[(C 2 H 5 ) 8 P] 2 PtCl 2 +CF 2 :CFLi _^ x > ci*-[(C 2 H 5 ) 3 P] 2 Pt(CF:CF 2 ) 2 (51%) 

Presumably this method could be adapted to the synthesis of perfluoro- 
alkyl complexes. 

2. From Perfluoroalkyl Iodides and Metal Carbonyls. In 1961 it was re- 
ported that perfluoroalkyl iodides react with pentacarbonyliron at moderate 
temperatures to yield iodo(perfluoroalkyl)tetracarbonyliron compounds, 
together with very small amounts of dimeric compounds that arise through 
thermal decomposition of the former products and are best prepared in 
this manner, 304,309 e.g., 

70° 
n-G 3 F 7 I+Fe(CO) 5 ^^ n-C 3 F 7 .Fe(CO) 4 I(80%)+[n-C 3 F,.Fe(CO) 3 I] 2 

75° 
2n-C 3 F 7 .Fe(CO) 4 I »■ [n-C 3 F,.Fe(CO) 3 I] l! (90%)+2CO 

The similarity between the reaction of a perfluoroalkyl iodide (GF 3 I, C 2 F S I, 
and n-C 3 F 7 I were used) with pentacarbonyliron and that between iodine 
and pentacarbonyliron, which yields Fe(CO) 4 I 2 , was noted at once and 
was ascribed to the pseudohalogen character of a perfluoroalkyl group. This 
led to the suggestion that those metal carbonyls known to react with 
iodine to give complex iodides should combine with a perfluoroalkyl iodide 

189 



Perfiuoroalkyl Derivatives of the Elements 

to yield perfluoroalkylmetal compounds. The following results* 1 * provided 
immediate support for this hypothesis : 



5i-C 5 H 5 .Co(CO) 2 +C a F 5 I j^— >- jr-C 5 H 5 .Co(CO)(C 2 F 6 )I(72%)+CO 

[(C,H s ) 2 P.CH 2 .CH 2 .P(C 6 H 5 ) 2 ]Ni(CO) 2 +n-C 3 F,I -^^> 

[(C 6 H s ) 2 P.CH 2 .CH a .P(C 6 H 5 ) 2 ]Ni(n-C 3 F 7 )I(84%)4-2CO 

[^-C 5 H 5 .Ni.CO] 2 +CP 3 I be °^" e > Jt-C 5 H 5 .Ni(CO).CF 3 

and the method has been used recently to convert dicarbonyl-ji-cyclopen- 
tadienylrhodium into compounds of type jr-C 5 H 5 Rh(CO) (R F )I (R F = CF S , 
C 2 F 6 , or n-C 8 F 7 ). 811 Triphenylphosphine displaces carbon monoxide from 
the cobalt compounds, e.g., 812 

50° 
jz-C 6 H 6 Co(CO)(C 2 F 6 )I +(C 6 H g ) 3 P benzene > CO +^-C 6 H 5 Co|:(O 6 H 5 ) 3 P](C 2 F 5 )I(60%) 

Following this, and with the knowledge that methyl-palladium bonds 
are cleaved by iodine to give methyl iodide and a palladium iodide, two 
perfluoropropyi derivatives of palladium were prepared as follows: 813 



tetrahydrofuran 

+ n-C s F 7 I — > 



f V (48%) ^^rT \j[ V (»%) 

N CH 3 I N CsFj-n 




/ 

3. From Unsaturated Fluorocarbons and Metal Carbonyls. A fair amount 
of effort has been devoted to the investigation of reactions between un- 
saturated fluorocarbons and transition metal compounds, the main objective 
being to determine if jr-complexes analogous to those derived from hydro- 
carbon systems can be obtained. The possibility that a perfluoro-olefin, 
despite its electron deficient nature (see p. 26), might be successfully 
jr-complexed to a transition metal was first suggested in 1959. It was point- 
ed out 314 that although replacement of hydrogen substituents in an olefin 
by fluorine would be expected to cause a substantial decrease in the donor 
power of the olefinic zr-bond, it ought to result in an increase in the acceptor 
character of the olefinic :r*-antibonding orbitals ; thus, for a perfluoro-olefin, 

190 



Transition Metals 

the ff-bond formed by donation of ^-electrons to a vacant metal orbital ought 
to be very weak but it should be supplemented by a strong ji-bond formed 
by overlap of filled metal d-orbitals with jr*-antibonding orbitals. 

A number of attempts to prepare jr-complexes from simple polyfluorinated 
olefins have resulted in the formation of only a-complexes. For example, 
reaction of tetrafluoroethylene with iron carbonyls 809, 81B or dicarbonyl-Tt- 
cyclopentadienylcobalt 814 yields the novel heterocyclic compounds (XXVIII) 
and (XXIX), respectively, while treatment with octacarbonyldicobalt gives 
the compound (CO) 4 Co.CF 8 .CF 8 .Co(CO) 4 , 81S - 8W which can also be prepared 
from tetrafluorosuccinyl chloride and sodium tetracarbonylcobaltate (c/. 
method 1, p. 188). <r-Bonded complexes [e.g., CHF s .CF 8 .Mn(CO) 5 , n- 
C s H 6 .Mo(CO) 3 .CF g .CHF 8> 5r-C 6 H 5 .W(CO) 3 .CF 2 .CHF 8 ] can also be obtained 
from tetrafluoroethylene and a variety of transition-metal earbonyl 
hydrides j 800,318 the material initially believed to be the ^-complex 
PtHCl(7r-C 2 F 4 ) [P(C 2 H 5 )3] 2 , isolated from the product of a thermal reaction 
between tetrafluoroethylene and «roMS-PtHCl[P(C a H 5 ) 3 ] 2 in Pyrex, 818 is 
now known to be <ro»s-PtCl(CO) [PfCjjHgy JX" (where X" = BFj and 
SiF^). 819 



CO 

CF.-CFrJ y QO ^i /CFj-CF, 

CF,— CF/ I N CO OC X X CF,— <3F, 

CO 
(XXVIII) (XXIX) 

However, tetrafluoroethylene complexes of iron, nickel, rhodium, and 
iridium are known from which the perfluoro-olefin can be displaced quite 
readily, 820 e.g., 

[(C 6 H 6 ) 3 P] 8 BhCl+C 2 F 4 CMOI °^ m > [(C,H f ) t P] i Rh(C,F 1 )Cl(70%) ^^ 

[(C e H 5 ) s F] 2 Bh(CO)Cl+C 2 F 4 
B^C H.C 

H0(_ _ Rh(O t H 4 J t +C 1 F 4 ► HC( Bh (>76%) -^-»> 

H,C H,C 

H,C 

HCi v BhfcQk+CjFi+CsH, 

,r>. — 0/ 



^>C— O' 



H,C 

191 



Perfluoroalkyl Derivatives of the Elements 

and in which the metal-tetrafluoroethylene bonds appear to have consider- 
able jr-character rather than consist of two metal-carbon cr-bonds. The latter 
type of bonding is believed to occur in compounds obtained by displace- 
ment of triphenylphosphine from tetrakis(triphenylphosphine)platinum by 
polyfluoro-olefins : S21 

benzene •*-* j 2 

[(C 6 H 5 ) s P] 4 Pt+CF 2 :CFX — ► [(C 6 H 6 ) 8 P] 2 Pt/ | 

X CFX 
(X = F, CI, or CF 3 ) 

Thermal reaction of dodecacarbonyltri-iron with perfluorocyclohexa-1,3- 
diene yields tricarbonyloctafluorocyclohexa-l,3-dieneiron (XXX) in which 
the diene residue appears to be bound to the metal via one n- and two 
a-bonds; 318,822 a similar situation seems to exist in some complexes derived 
from perfluorobut-2-yne [e.g., 328 (XXXI)]. 








B. Properties 

In general perfluoroalkyl derivatives of the transition metals are air- 
stable, volatile substances, which occur in a variety of colours [e.g., Bipy. 
Pd(n-C 3 F 7 ) 2 , m.p. 180-181°, very pale yellow; (n-C 3 F 7 ) 2 Fe(CO) 4 , m.p. 89°, 
pale yellow; C 2 F 5 .Fe(CO) 4 I, m.p. 15°, purple-red; ?r-C 5 H 5 .Co(CO) (C 2 F 6 )I, 
m.p. 138°, black] . In general, the a-complexes are much more stable than 
their hydrocarbon counterparts towards oxygen, moisture, and heat. 800 
Thus, CF 3 .Co(CO) 4 is stable at its boiling point (91°), whereas CH 3 .Co(CO) 4 
decomposes above 0°; (C 2 H 6 ) 2 Fe(CO) 4 is apparently unknown, but 
(C 2 F 5 ) a Fe(CO) 4 exists and is unchanged at temperatures up to 100°; 
C 2 F 5 .Mn(CO) 8 decomposes only at temperatures above 120°, while the 
existence ofC 2 H 6 .Mn(CO) 8 isdoubtful;7r-C 5 H s .Ni(CO).CF 3 is quite stable at 
room temperature, in contrast tojr-C 8 H 6 .Ni(CO).CH 3 , which rapidly decom- 
poses to a black solid at ambient temperatures. Pyrolysis of CF 3 .Fe(CO) 4 I 
at temperatures above 100° appears to yield difluorocarbene. 804 

It has been suggested 824 that the relatively great thermal and aerobic 
stability of perfluoroalkyl derivatives of transition metals results from 
extra stabilisation of a carbon — metal cr-bond in such a compound through 
the ability of a highly electronegative fluorocarbon group to cause the metal 
bonding orbital to contract and thus overlap more effectively with the carbon 

192 



Beferences 

orbital, coupled with a 71-interaction involving overlap of filled metal d„ 
orbitals with empty a G— F a* antibonding orbitals. In valence bond termi- 
nology the situation in a trifluoromethyl derivative, for example, has been 
described by reference to the contributions of canonical forms (A)— (E) 

F F F F F~ 

I I I ! 

F— CT M + *-» F— C— M •*-»• F" C=M + «->■ F— C=M + ■*-+ F— C^M* 

III I . 

F F F F" F 

(A) (B) (C) (D) (E) 

(M = transition metal atom with attendant Uganda) 

which implies that the — M bond order should increase while that of an a 
C — F should decrease. Persuasive evidence for these changes in bond order 
can be quoted.* 25 For example, X-ray diffraction studies have revealed 
that the carbon — molybdenum bond length in jr-C s H 6 .Mo(CO) s .C 3 F 7 -n is 
significantly shorter than in an anlogous non-fluorinated complex, 
3i-C 5 H 5 .Mo(CO) 3 .C2H B ; and detailed investigation of the infrared spectrum 
of CF 3 .Mn(CO) 5 has shown that the C — F stretching force constant is 
1-3 md./A lower in this compound than in trifluoroiodomethane [&(CF) calc. 
= 5-9 md./A], which corresponds to about a 20% decrease in C — F bond 
orders. The ready isomerization of polyfluoroallyl derivatives of manganese 
has been quoted as a chemical consequence of the weakness of the a C — F 
bonds and the resulting presence of internal nucleophiles : 386 



*G C 

F [V * X x /Mn(CO) s 

— *■ 2 >C=C< 
F F \ F OF/ N? 

C Jr- ° 
(OCJsMn^^C^^F 



(X = ForCl) 



BEFERENCES 

The following detailed reviews are available: Lagowski, Quart. Rev., 1959, 13, 
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193 



Perfluoroalkyl Derivatives of the Elements 

of Li, Mg, Zn, Hg, B, Al, Si, Ge, Sn, Pb, N, P, As, Sb, S, Se, CI, Br, I, and 
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78. Clifford and Dun-can, Inorg. Chem., 1966, 5, 692. 

79. (a) Babr and Haszeldine, J. Chem.. Soc, 1955, 2532. 

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80. Banks and Moore, J. Chem. Soc. (G), 1966, 2304. 

81. Banks, Cheng, and Haszeldine, J. Chem. Soe., 1964, 2485. 

82. Banks, Haszeldine, and Hatton, J. Chem. Soc. (C), 1967, 427; Banks and 
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83. Greenwood and Hooxon, J. Chem. Soc. (A), 1966, 751. 

84. Barb and Haszeldine, J. Chem. Soc, 1956, 3428; Young, Durrell, and 
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85. Banks, Berry, and Moore, J. Chem. Soc. (C), 1969, 2598. 

86. Moissan, Ann. Chim. (Phys.), 1891, 24, 262. 

87. Ruff and Giese, Ber., 1936, 69B, 598, 604, 684. 

88. Ruff and Willenberg, Ber., 1940, 73B, 724. 

89. Young and Dresdner, J. Org. Chem., 1963, 28, 833. 

90. BtRCHALL, Ellis, Fields, and Haszeldine, private communication. 

91. Robson, McLoughlin, Hynes, and Bigelow, J. Amer. Chem. Soc, 1961, 83, 
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92. Emeleus and Hurst, J. Chem. Soc, 1962, 3276. 

93. Hynes, Bishop, and Bigelow, Inorg. Chem., 1967, 6, 417. 

94. Coates, Harris, and Sutcliffe, J. Chem. Soc, 1951, 2762. 

95. Cuculo and Bigelow, J. Amer. Chem. Soc, 1952, 74, 710; Haszeldine, 
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96. Bishop, Hynes, and Bigelow, J. Amer. Chem. Soc, 1962, 84, 3409. 

97. Gebvasi, Brown, and Bigelow, J. Amer. Chem. Soc, 1956, 78, 1679. 

98. Thompson and Emeleus, J. Chem. Soc, 1949, 3080. 

99. Haszeldine, J. Chem. Soc, 1951, 102. 

100. Avonda, Gervasi, and Bigelow, J. Amer. Chem. Soc, 1956, 78, 2798. 

101. Attaway, Groth, and Bigelow, J. Amer. Chem. Soc, 1959, 81, 3599* 

102. Haszeldine, J. Chem. Soc, 1950, 1966. 

103. Koshar, Husted, and Meiklejohn, J. Org. Chem., 1966, 31, 4232. 

104. Koshar, Husted, and Wright, J. Org. Chem., 1967, 32, 3859. 

105. Kauok and Simons, U.S.P. 2,616,921/1953. 

106. Simons, U.S.P. 2,490,099/1949. 

107. Simmons et al., J. Amer. Chem. Soc, 1957, 79, 3429. 

108. Kauck and Simons, U.S.P. 2,631,151/1953. 

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113. Belcher and MaoDonald, Mikrochim. Acta, 1956, 1111; Kakabadse and 
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114. Pearlson and Hals, U.S.P. 2,643,267/1953. 

115. Banks, Cheng, and Haszeldine, J. Chem. Soc, 1962, 3407. 

116. Mitsoh, J. Amer. Chem. Soc, 1965, 87, 328. 

117. Banks and Williamson, J. Chem. Soc, 1965, 815. 

118. Banks and Burling, J. Chem. Soc, 1965, 6077. 

119. Banks, Ginsberg, and Haszeldine, J. Chem. Soc, 1961, 1740. 

120. See ref. 118 and RebeRtus, McBrady, and Gagnon, J. Org. Chem., 1967, 32, 
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121. Banks, Burling, and Hatton, unpublished results. 

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134. Haszeldine, J. Chem. Soc, 1953, 2075. 

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166. Bennett, Emeletts, and Haszeldine, J. Chem. Soc, 1953, 1565. 

167. Emeleus and Smith, J. Chem. Soc, 1959, 375. 

168. Haszeldine and West, J. Chem. Soc, 1956, 3631; ibid., 1957, 3880. 

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171. Bubg and Sabkis, J. Amer. Chem. Soc, 1965, 87, 238. 

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176. Mahleb, J. Amer. Chem. Soc, 1968, 90, 523. 

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182. Habbis, J. Chem. Soc, 1958, 512. 

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187. Gbieeiths and Bubg, J. Amer. Chem. Soc, 1960, 82, 1507. 

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308. Rest, Roseveab, and Stone, J. Chew. Soc (A), 1967, 66. 

309. Manuel, Staffobd, and Stone, J. Amer. Chem Soc, 1961, 88, 249. 

310. King, Tbeichel, and Stone, J. Amer. Chem. Soc, 1961, 83, 3593; McBbide, 
Staffobd, and Stone, 1963, 723. 

311. McClevebty and Wilkinson, J. Chem. Soc, 1964, 4200. 

312. Tbeichel and Webbeb, Inorg. Chem., 1965, 4, 1098. 

" 201 



Perfltioroalkyl Derivatives of the Elements 

313. Maitlis and Stone, Chem. <fc Ind., 1962, I860. 

314. Watterson and Wilkinson, Chem. & Ind., 1959, 991. 

315. Hoehn, Pratt, Watterson, and Wileinson, J. Chem. Soc, 1961, 2738. 

316. Coyle, King, Pitcher, Stafford, Treichel, and Stone, J. Inorg. Nuclear 
Chem., 1961, SO, 172. 

317. Booth, Haszeldine, Mitchell, and Cox, Chem. Comm., 1967, 629. 

318. Clare and Tsang, J. Amer. Chem. Soc., 1967, 89, 529 and references quoted 
therein. 

319. Clare, Corfield, Dixon, and Ibers, J. Amer. Chem. Soc., 1967, 89, 3360. 

320. Mays and Wilkinson, J. Chem. Soc, 1965, 6629; Cramer and Parshall, 
J. Amer. Chem. Soc, 1965, 87, 1392; Parshall and Jones, ibid., p. 5356; 
Fields, Germain, Haszeldine, and Wiggans, Chem. Comm., 1967, 243; 
Kemmitt and Nichols, ibid., p. 919. 

321. Green, Osborn, Best, and Stone, Chem. Comm., 1966, 502. 

322. Churchill and Mason, Proc. Chem. Soc, Lond., 1964, 226. 

323. Boston, Sharp, and Wileinson, J. Chem. Soc, 1962, 3488; Bailey, Ger- 
looh, and Mason, Nature, 1964, 201, 72. 

324. Wilford and Stone, Inorg. Chem., 1965, 4, 389; Cotton and McCleverty, 
J. Organometal. Chem., 1965, 4, 490; Stone, Endeavour, 1966, 26, 33. 

325. Churchill and Fennessey, Inorg. Chem., 1967, 6, 1213; Clark and Tsai, 
J. Organometal. Chem., 1967, 7, 515; Cotton and Wing, ibid., 1967, 9, 511. 

326. Goldwhite, Rowsell, and Valdez, J. Organometal. Chem., 1968, 12, 133. 



202 



ChaptebS 
PERFLUORINATED AROMATIC COMPOUNDS 



The chemistry of perfluorinated aromatic compounds 1 has been developed 
almost entirely since 1955, the year in which the first practical synthesis 
of hexafluorobenzene was disclosed. Progress was slow at first owing to 
preparative difficulties, but accelerated rapidly during the early 1960's when 
hexafluorobenzene and other basic starting materials became available 
commercially ; much effort is still being spent in this area of fluorocarbon 
chemistry and current trends indicate a growing interest in heterocyclic 
compounds (e.g., pentafluoropyridine, tetrafluoropyrimidine). Although 
more than sixty polyfluorinated aromatic compounds can be purchased in 
development quantities, 2 no significant commercial outlet for any member 
of this class has been discovered yet ; searches are being made for products 
useful as special-purpose polymers, lubricants, fluids, pharmaceuticals, 
agricultural chemicals, dyestuffs, or rubber additives. 3 



I. AEOMATIC FLUOROCARBONS 

A. Preparation 

Pluorination of aromatic hydrocarbons by methods which lead to com- 
plete replacement of hydrogen by fluorine also results in saturation of the 
double bonds, so that only the corresponding perfluoro-alicyclic compounds 
are obtained, together with fluorocarbon degradation products (see Chap- 
ter 2). The standard indirect method for the preparation of aromatic fluoro- 
compounds, namely the Balz-Schiemann reaction 

(Ar.H >- Ar.NOj > Ar.NH 2 > Ar.N 2 +BF 4 - > Ar.F+N 2 +BF s ), 

has been used to effect step-wise replacement of four hydrogen atoms in 
benzene by fluorine ; 4,8 but extension of this method to give pentafluoro- 
benzene proved impossible owing to the expulsion of two para-fluorine 
atoms, to give 2,5-difluorobenzoquinone, during the attempted nitration 
of the tetrafluorobenzene.* 

The first report on the synthesis of aromatic fluorocarbons to be published 
appeared in the literature in 1947 ; 6 it contained details of the preparation 
of hexafluorobenzene and octafluorotoluene in low yields by step-wise 

•i** 203 



Perfluorinated Aromatic Compounds 

fluorination of hexachlorobenzene and 1,2,3,4,5-pentacUorotrifluoromethyl- 
benzene, respectively, followed by dehalogenation of the products, e.g., 

BrFa . . . SbFj, chlorofluorocarbon solvent 
CjCl,, ^ „> C 6 Br a Cl 4 F 8 (average composition) — >■ 



0-150" 



60° 



Zn dust, ethanol _„, .... , /-»/-«» 

C 6 BrCl 4 F 7 (average composition) — > C 8 F 6 (ca.5% yield based on C,C1„) 

Studies on the samples of these aromatic fluorocarbons prepared in this 
way appear to have been restricted to measurement of their physical 
properties, 6 including an examination of the u.v. absorption spectrum of 
octafluorotoluene, 7 and hydrolysis of octafluorotoluene to pentafluoro- 
benzoic acid with hot concentrated sulphuric acid. 8 Perhaps the work was 
abandoned due to difficulties encountered in the preparation of sufficient 
material for systematic studies : a more recent and detailed report on the 
fluorination of hexachlorobenzene and other halogenoaromatic compounds 
with bromine trifluoride states that the reactions are not readily controlled 
and explosions frequently occur. 9 

Detailed study of an aromatic fluorocarbon was at last made possible 
by the disclosure in 1955 that hexafluorobenzene can be obtained in reason- 
able yield by pyrolysis of the readily-accessible tribromofluoromethane : w ' u 



CBr 4 



SfrF, activated with Br, 



120-130° 



> CFBr s (75%) 



830-640° 

> 

Pt tube 



C„F 6 <45%) 



This method (which was discovered by Mile. Y. Desirant working in Swarts' 
laboratory in the mid-1930's!) has been much used to prepare hexafluoro- 
benzene; 1213 it suffers from the disadvantage that the weight yield of the 
product is poor, since 90% of the tribromofluoromethane is lost as bromine. 
The yield of hexafluorobenzene is increased to 55% if tribromofluoro- 
methane is pyrolysed at 4-5 atm pressure. 12 

Several other methods of preparation of hexafluorobenzene have been 
reported since 1"955 ; x the most important of these are (i) defluorination of 
a mixture of octafluorocyclohexa-1,3- and -1,4-diene obtained by dehydro- 
fluorination of decafluorbcyelohexanes prepared by fluorination of benzene 
with cobalt trifluoride (see p. 10) : 14 



HF 

il Jhf 



F 2 




F 

Fsf^^F 



strong KOH aq. 



F 



heat 



(90%) 




400-500° * F \ 



(90%) 



F 



204 



Aromatic Fhtorocarbons 

and (ii) reaction of hexachlorobenzene with anhydrous potassium fluoride 
at elevated temperatures : 15 



CI 



IIiTNc 



Clf ^|C1 KF F|| 



cilyci 

CI 



450-500° 



F 



F 



CI 

F Frf^^NF 

(21%), (20%), 

F Fl Jf 

F 



CI 
Fi| ^;F 



CI 



(14%), F | f (12%) 

F>1 VC1 Oil JCI 



F 



F 



The discovery of the defluorination route (i), which has been developed 
commercially, 1 led to the introduction in 1959 of the first general method 
of synthesis of aromatic fluorocarbons> viz., complete fluoridation of an 
aromatic or alicyclic hydrocarbon with cobalt trifluoride (see p. 10) 
followed by aromatization of the resulting saturated alicyclic fluorocarbon 
with hot finely-divided nickel or, preferably, iron, e.g., 16-19 



CH 3 




F CF 3 



CoF, F. 



300-820° 



F a l 



CF 3 



Fe F(T ^iF 
(70%) -ioF > I' I (25%) 



F^ VF 
F 




F C S F 5 



CiF 5 



CoF, Fji 



800-320° 



2 Fe Flf 

2 (54%) ^-> " 



490° 



(23%) 



F, 



F 



600° 



Fe 



CF=CF. 

FiT^^lF 




(15%) 



F 1 ^ J® 
F 



205 



Perfluorinated Aromatic Compounds 




\ r\ 



Sj F 2 j, Fj F 2 

C0F V »/>/>. <«%> Fe 



800-360° 



^ / F ^ ' 

F 2 Fg F 2 F 2 





X 



(saturated soln. 
in tetralin) 



F F 



CoF, F ! 



350° 




F 2 Fe Ff 

> 



"V 



[F 



F2 \./ A \// Fa 500 ° F \/\/ F 

F 2 F 2 



(51%) 



F 




Fa r* 2 

jIf ' f 



-1 coFa F 2 r 

J 400 ° F 2 l 




F2 F2 

F. 



F * ( 48%) -^* 
j F 500" 



1 . , 

Fk >\^'F 
F F 




(1%) + 



fAAf 



(10%) 



The only other general method known stems from the halogen-exchange 
reaction (ii) above, which was reported in 1963, and differs from the fluorina- 
tion/defluorination route in that it can be applied to the preparation of 
perfluorinated aromatics containing familiar functional groups like — CN, 
and— COF, e.g., 80 ' 81 



01 CI F F 

*lCl KF, 450° (autoclave) Fr^ Tf ^iF 




isi, or KF to tetramethylene ■„! u i— 

^ l sulphone, 285° * X^,/ \/ 

CI 01 F F 



(50-60%) 



CI F 

Oli^NcN Kg, 250° Fjj^'NoN 

Oil -JcN antoolave f'1 ^JcN 

a f 



(69%) 



Hexakis(trifluoromethyl)benzene can be prepared by the trimerization 
of perfluorobut-2-yne (see p. 61). 



206 



Aromatic Fluorocarbons 

B. Properties and Reactions of Aromatic Fluorocarbons 
and their Derivatives 

1. Physical Properties. The reported physical constants of some aromatic 
fluorocarbons are listed in Table 5.1, together with those of their hydro- 
carbon analogues. With the exception of perfluoroacenaphthylene, a yellow 
solid, m.p. 149 °, M the known aromatic fluorocarbons, like their aliphatic 
relatives, are colourless compounds that do not support combustion and 

Table 5.1. Comparison of Some Physical Properties of Aromatic Fluorocarbons with 
those of Their Hydrocarbon Analogues 



Compound 


m.p. (°C) 


b.p. (°C) 


n' D (t°C) 


df 4 (t°C) ' 


Ref. 


Perfluorobenzene 
Benzene 


5-3 
5-5 


80-5 
80-1 


1-3777 (20) 
1-5011 (20) 


1-6184 (20) 
0-8790 (20) 


11 
22 


Perfluorotoluene 
Toluene 


< —70 
— 95 


102-103 
110-6 


1-3680 (19) 
1-4969 (20) 


1-660 (25) 
0-8669 (20) 


6,16 
22 


Perfluoro(ethylbenzene) 
Ethylbenzene 


— 93-9 


114-115 
136-2 


1-3620(19) 
1-4983 (14-5) 


0-8669 (20) 


16 
22 


Perfluoro-o-xylene 
o-Xylene 


— 29 


128 
144-4 


1-3670 (19) 
1-5055 (20) 


0-8802 (20) 


23 

22 


Perfluoro-p-xylene 
p-Xylene 


13-2 


122 
138-4 


1-3621 (17) 
1-4958 (20) 


0-8611 (20) 


23 

22 


Perfluorobiphenyl 
Biphenyl 


67-69 
69-71 


254-255 


1-5882 (77) 


0-9919 (73) 


16 
22 


Perfluoronaphthalene 
Naphthalene 


86-87 
80-2 


217-9 


1-5822 (99-6) 


1-145 (20) 


16 
22 


Perfluoro(2-methyl- 

naphthalene) 
2-Methylnaphthalene 


42-5-43 
35-1 


245 


1-6026 (39-9) 


1-029 (20) 


24 
22 



which are much more volatile than would be expected on the basis of their 
molecular weights. A close correspondence exists between the melting 
and/or boiling points of the known aromatic fluorocarbons and those of 
their hydrocarbon counterparts; the fluorocarbons have lower refractive 
indices and higher densities than the hydrocarbons. Most of the aromatic 
fluorocarbons so far prepared do not show the extremely limited solubility 
in, or miscibility with, common organic solvents that is characteristic of 
aliphatic fluorocarbons (see p. 16). 

Hexafluorobenzene, the most extensively studied compound of its class, 
is a colourless liquid with a sweet odour; it seems to be non-toxic but 
possesses anaesthetic properties. 1 It is miscible with many common organic 

207 



Perfluorinated Aromatic Compounds 

solvents and immiscible with water; it forms 1:1 molecular complexes 
with benzene (m.p. 23-7°), mesitylene (m.p. 34°), and 2-methylnaphthalene 
(m.p. 56°), those with the last two hydrocarbons being sufficiently stable 
to allow their recrystallization from ether. 28 Hexafluorobenzene shows high 
thermal stability and good resistance towards radiation. Thus it undergoes 
only slight decomposition when stored in a Nimonic (M/Cr alloy) vessel 
at 500° for 3 weeks, 1 but breaks down to yield perfluoroethane, perfluoro- 
toluene, and perfluorobiphenyl when passed through a platinum tube 
heated to 850 . 11 Irradiation of hexafluorobenzene with u.v. light converts 
it slowly into hexafluorobidyclo[2,2,0]hexa-2,5-diene [the 'Dewar' form (I)] 
and polymeric material: 26 



u.v. light F /f^/ ^^f F 

(15%) + polymer (5%) + 





■p /Z^^^F ij hexafluorobenzene (80%) 

(vapour diluted with N 2 or Ar) 

The para-bonded isomer (I), b.p. 52°, is apt to explode violently if stored 
as a neat liquid but can be kept safely as a solid at temperatures below 
— IS . 263 It reverts quantitatively to hexafluorobenzene when heated at 
80° for 4 hr. and undergoes many reactions typical of aperfluoro-olefin; 26d 
however, it can also act as a powerful dienophile in the Diels-Alder reaction, 
e.g., 868 



20° 





[X = CH 2 , O, NH, C:C(CH3) a , or (CH^] 

The general trend in the elementary physical properties of mohofunc- 
tional derivatives of hexafluorobenzene (see Table 5.2) is the same as that 
mentioned above for aromatic fluorocarbons. The marked difference in 
boiling point between phenol (182°) and pentafluorophenol (143°) is caused 
by decrease in intermolecular hydrogen bonding, arising from the decreased 
basicity of the oxygen caused by the adjacent electron-attracting penta- 
fluorophenyl group. 13 The ionization constant of pentafluorophenol in 
water (K a = 3-0 x 10" 6 at 25°) is distinctly greater than that of phenol 
(K a = 1-3 x 10- 10 at 25°) or the fluoro-alcohol (n-CsF^CH.OH (K a = 30 
x 10 -11 at 25°), and approaches that of benzoic acid (K a = 6-3 x 10 -5 

208 



Aromatic Fluorocarbons 

Table 5.2. Comparison of the Physical Properties of Some Hexaflnorobenzene 
Derivatives with those of the Corresponding Benzene Derivatives 





C 6 F 5 .R 


C,H S .R (Ref. 22) 




m.p. 


b.p. 






m.p. 


b.p. 




R 


(C°) 


(°C) 


n'j,(t°C) 


Ref. 


(°C) 


(°C) 


n' D {t°cy 


H 


— 


85 


1-3931 (18) 


27 


5-5 


80-1 


1-6011 (20) 


F 


5-3 


80-5 


1-3777 (20) 


11 


—41-9 


84-9 


1-4646 (22-8) 


CI 


— 


122-123 


1-4256 (20) 


6 


— 45 


132 


1-5248(20) 


Br 


— 


134-135 


1-4505(19) 


28 


— 30-6 


155-156 


1-5598 (20) 


I' ' 


— 


161-163 


1-4970(19) 


28 


— 31-4 


188-6 


1-6216 (18-5) 


NO 


44-5 


— 


— 


29 


68 


— 


, — :•■■.- 


N0 2 


— 


158-160 


— 


30 


5-7 


210-9 


1-5529(20) 


NHj 


34 


153-154 


— 


31 


— 6-2 


184-4 


1.5863 (20) 


NH.NH, 


77-78 


— 


— 


32 


19-6 


— 


1-0978 (20) 


OH 


38-5-39-5 


143 


1-4270 (20) 


13,33 


41 


182 


1t5425 (40-6) 


SH 


— 


143 


1-4622 (22) 


34 


— 


169-5 


1-5861 (23-2) 


O.CHj 


— 


138-139 


1-4090 (20) 


33 


— 37-3 


155 


1-5179 (20) 


CH S 


— 


117-118 


1-4023 (20) 


35 


— 95 


110-6 


1-4969 (20) 


CHO 


20 


168-170 


1-4505 (17) 


28,36 


— 26 


179-5 


1-5463 (17-6) 


C0 2 H 


101-5-102 


— 


■ — 


37 


122 


249 


1-5397 (15) 



at 25°), thus indicating the effect of conjugation in the aromatic ring. 3 ? 
It also reveals the extent to which back-coordination with electron-release 
from fluorine, 39 e.g., 



OH 



OH 




+ F 



offsets the inductive effect of the aryl fluorines, since pentachlorophenol is 
more acidic (K a = 5-5 x 10 _s ). Proton magnetic resonance studies indicate 
that the electron- withdrawing capacity of pentafluorophenyl lies between 
that of chlorine and bromine, and measurements of the recoilless emission 
and absorption of y-rays (Mossbauer spectroscopy) in tetrakispentafluoro- 
phenyltin suggest that the electronegativity of the C 6 F 8 group is approxi- 
mately the same as that of bromine. 40 The marked difference in acidity be- 
tween the pentafluorophenylmethanes (C 6 F 5 ) 3 CH (y>K a = 15-8 on the Streit- 
wieser scale) and (C 6 3? s ) a CH 2 (pX„ = 21-2), and between each of these and 
its hydrocarbon counterpart [(C 6 H 6 ) 3 CH, pl£ a =31-5; (C 6 H S ) 2 CH 2 , 
j>K a = 33-1], is ascribed primarily to the strong inductive influence of the 
C e F 5 group, since resonance stabilization of the conjugate base (C g F 6 ) 3 C - 

209 



Perfluorinated Aromatic Compounds 

should be minimal owing to lack of coplanarity caused by steric effects 
and relief of C — F dipole — dipole repulsions [the relatively small difference 
between the acidities of tri- and di-phenylmethane is ascribed to marked 
steric inhibition of resonance in (C 6 H S ) 3 C - ]. 41 

The i.r. spectra of hexafluorobenzene and its derivatives strongly in- 
dicate aromatic character in the cyclic triene system present. All the spectra 
show strong C— F stretching bands near 1000 cm -1 , and a strong band near 
1520 cm -1 that is attributed to the C=C ring vibration. 

2. Preparation ol Pentafluorophenyl Derivatives, (a) By Nucleophilic 
Attack on Hexafluorobenzene. Hexafluorobenzene, like a perfluoro-olefin 
(see p. 26), is susceptible to attack by nucleophilic reagents, and consider- 
able use has been made of this to effect the synthesis of pentafluorophenyl 
derivatives (see Fig. 5.1): 

F 

Fif^ |F Fi 

Nu _ + > 

F^^'F F 1 

F F 

(Nu- = H", HO-, HS", CHf, CH 3 .0", H 2 N", etc.) 

This type of reaction has also been employed to obtain polyfluoroaryl 
derivatives of transition metals and heterocyclic fluoro-compounds, e.g., 42, * s 

C,F, + Na+[^.C 5 H 5 Fe(CO) 2 ]- **"»»*■*"», C^FetCO^-C^^To) 




heat 



F F 



/NH.CH 2 .CH a .OH 



J J + H a K.CH,CH,OH ^ ~', J | f <«%) 



F 



K 2 CO a> heat F] 

dimethyUormamide p 




F 



(6) By Electrophilic Attack on Pentafluorobenzene. Neither hexafluoro- 
benzene nor any other aromatic fluorocarbon would be expected to undergo 
electrophilic substitution, since this would require the elimination of 
fluorine as a cation, F+. [The standard heat of formation of the gaseous 
F+ ion is 420 kcal/g ion, an extremely high value compared with those of 
other positive halogen ions (C1+, 327; Br+, 301 ; I+, 268 kcal/g ion), which 
means that fluoronium ions, even in solvated form, are very unlikely to 

210 



Aromatic Fhiorocarbons 






X w la X 



S 



fa. Uh 



O-rKl^ 



K> *o 




Perfluorinated Aromatic Compounds 

be formed in a chemical reaction.] Electrophilic reagents can be used suc- 
cessfully to hydrolyse perfluoro(methylbenzenes) to the corresponding 
perfluoro-acids, e.g., 8 ' 83 

CF S co 2 H 

|F cone. HaSO. Ff'^SF 

*A^J* (25%) 

F 

JL /C0 2 H 
fuming H,S0 4 W ^f 

and to convert pentanuorobenzene into pentafluorophenyl halides, 28 penta- 
fluorobenzenesulphonie acid, 28 pentafluoronitrobenzene, 44 or pentafluoro- 
phenylmethanes : 4S 

Br«, AlBr,, fuming H 2 SO, 




H 


/ 60-65° " ^'6^'^1/oJ 


X 


/ fuming HiS0 4 


FrT >F 


/ 15° *" c bF b .«>OsH (92%) 


F \X^ F 


\ fuming HNOs, BF,, 60-70° 


F 


\ tetramethylene sulphone °« 1 ' * MO * {ii ^ /o > 




\ CHO..A1C1. 




150°, autoclave * (CeF.JsCH (92 %) 



Pentanuorobenzene is produced commercially, together with hexanuoro- 
benzene and the three isomeric tetrafluorobenzenes, by the three-stage 
process: 1 

partial fluorination 
benzene __>. polyfluorocyclohexane mixture (C^a^Hj,, a = 0— 4) 

dehydroBuoiination 
*■ polyfluorooyolo-hexenes and -hexadienes 



defluorination 
j» C,F , CjF B H, C„F 4 H a . 



(c) From Pentafluorophenylrnagnesium Halides or Pentafluorophenyl- 
lifhium. Grignard reagents can be prepared without much difficulty from 
chloro-, 46 ' 47 bromo- 28 (this is the most popular), and iodo-pentafluoro- 
benzene;* 8 they undergo the standard reactions associated with such 

212 



Aromatic Fluorocarbone 

reagents and have been used to prepare a wide range of pentafluorophenyl 
derivatives, including those of other elements, e.g. (see also Fig. 5.2), 48-60 

¥gfir Zither > ; . ^^ ^ 

/ T1C1,, ether • , ■■ 

-> (C,F 5 ) t TlBr (45%) 




heat 



Mn(CO) s Br; e(her . _ ' . , ... 

:. ■ ' — —> C,F5.Mn(CO) 6 (47%) 



and of acetylene (note that C a F 6 .C':CF has not yet been reported), e.g., 81 
2C,F B .MgBr + 10=01 CoC |»»» tlwr > c,F 5 .C;C.C,,F 5 (56%) + 2MgBrI 

Pentafluorophenyl-lithium has also been used extensively to prepare penta- 
fluorophenyl derivatives of the elements, e.g./ 2-64 



j. CdCl«, ether 



->- (C,F 5 ) 2 Cd(51%) 



Ffi n^ 1 / SC1», ether-hexaae 

J J F \ n -78 to 2C, > < C « F ^ S ( 80% > 

F \ ^.[(C > H.y 3 ,PdCl, I |c tHi)lB]iM(q|F ^ (lrQIM .,74% i e«..8l6) 

ether, -80 to 20 + ^^.^H^pj^^c^C! (9 o /o) 

Pentaflnorophenyl-Uthium is preparedi by treatment of bromopentafluoro- 
benzene with n-butyl-lithium or lithium amalgam, 66 or, preferably, directly 
from pentaflizorobenzene by installation with n-bntyl-Hthium. 56 The last 
method yields pentafluordphenyl-lithitim rapidly and quantitatively at low 
temperatures in ether owing to the high acidity of pentafluorobenzene, 
which undergoes rapid hydrogen isotope exchange even in a dilute solution 
of sodium methoxide in methanol at room temperature. 57 

A synthesis involving pentafluorophenyl-lithium is effected, as indicated 
above, by adding pentafluorobenzene or bromopentafluprobenzene to a 
stirred solution of n-butyl-lithium in ether (or other solvent, e.g., ether — 
tetrahydrofuran, ether— hexane) at ca. — 70 9 and then allowing the resulting 
solution to warm to room temperature in the presence of the second reagent. 
In the absence of a second reagent, the pentafluorophenyl-lithium eliminates 
lithium fluoride at temperatures around 0° to generate tetrafluorobenzyne, 88 
a powerful dienophile that has been trapped with several substrates, in- 
cluding benzene and thiophene (see below) ; 89 in the absence of a trap the 
tetrafluorobenzyne reacts with undecomposed pentafluorophenyl-lithium 
to yield nonafluoro-2-lithiobiphenyl, which can be detected by addition 

213 



Perfluorinated Aromatic Compounds 






fa- 
ll 



fa 




o 



fa- 



IS 

Si 



IS s 



X 
u 



fa 



IS 



« 



§ 



a 
o 



o 



s? 



•3 

3 



s 



a 
o 



fa 







B 






o 
o 


>^ 


»? 


O 








S*i 


£* 


u 






ss 


o 




a 

O 

a 
-> 9 






9 




o 


,y 


-z 


fa 


tC 


» 


&. 


K 


c 


o 


u ■> 




u 


X. 








B 



fa 
u 



"8 

e 
s 








s 








Si 




* 


fa' 




c" 


n. 






o 


*. 


o 


s 


fa 


*. 



=0 
IM* 

id 

6 

tH 
f6| 



214 



Aromatic Fluorocarbons 

of water to the system and isolation of the hydrolysis product 2£?-nona- 
fluorobiphenyl. Pentafhiorophenylmagnesium bromide is largely unaffected 
when heated under reflux in ether, 86 but at higher temperatures it too 
decomposes to yield tetrafluorobenzyne; thus the 1,4-adduct of benzene 
and tetrafluorobenzyne is formed when pentafluorophenylmagnesium 
bromide is heated under reflux with benzene after removal of the original 
solvent ether. 47 



F 
F 



F, 



MgBr 



\Jp 




Li 




(d) Free-radical Reactions. Very few results pertaining to homolytic sub- 
stitution of fluorine in hexafluorobenzene have been published. Arylation 
of hexafluorobenzene to give 2,3,4,5,6-pentafluorobiphenyl or an appropriate 
derivative as the major product can be achieved by thermal reaction 
with nitrobenzene (C 6 H 6 .N0 2 ^ C 6 H 8 . + N0 2 ) M or an aroyl peroxide 

[(X-C^.COaOa 2£*£* 2X.C 8 H 4 .CO.O ► 2X.C 6 H 4 - + 2CO a ; X = H, 

3-CH 3 , 3-C1, 3-Br, or 4-NOJ : 8l 



F 
F 




+ RF 



[B- = radical present in the system, e.g., X.C s H 4 .CO.O-, X.C 6 H 4S or another cr-com- 
plex (A) in the case of peroxide arylation] 



and alkylation with photochemical sources of trifluoromethyl radicals 
[(CF 3 ) 2 C:0 or CF 3 I/u.v. light 62 ] gives perfluorotoluene in low yield. Ultra- 



215 



Perfhwrinated Aromatic Compounds 

violet irradiation of hexafluorobenzene with trichlorosilane gives dichloro- 
fluoro(pentafluorophenyl)silane in 61% yield; this product presumably 
arises from reactions undergone by the tr-complex (B) formed initially, 
possibly with the elimination of atomic chlorine and propagation of the 
chain: 68 

u. v. light 
SiHOU *■ Cl 8 Si-+H> 

Cl 3 Si F SiFCl 2 



Fff^hF Frfi-JF FrJ^^iF 

3i +• — ► ' • I — *■' 

vK^y* f^^'f ^V/*" 



SiHCl, 



HCi+ca 3 si- -^ 



Photochemical chlorination of hexafluorobenzene yields 1,2,3,4,5,6-hexa- 
chlorohexafluorocyclohexane. 64 

Pentafluorophenyl radicals can be generated by oxidation of pentafluoro- 
phenylhydrazine in non-aqueous media, 65 by pyrolysis of pentafluoro- 
benzenesulphonyl chloride, 88 and from pentafluoroiodobenzene by photo- 
lysis, 67 pyrolysis, 87 or thermal reaction with nickel tetracarbonyl. 88 Examples 
of the use of these methods in synthesis are given below. The 'direct' route 
to pentafluorophenyl derivatives of the elements (C 6 F 6 I/element/heat), 
which parallels the classical route to perfluoroalkyl derivatives (see Chap- 
ter 4), was developed only recently, 63,67 ' 69 presumably owing to the ease 
of formation and reactivity of the lithium and magnesium intermediates 
C 6 F B Li and C 6 F 5 .MgBr (see p. 212). 

HgO, He 

-> (C 6 F B ) 2 Hg (64%) 



/ perfiuoro-n-pentane 
C,F B .NH.NH 2 — < 

V_O^0A_^ C6F5 . C6H5(74%) 

CI 
1,3,5-0I,C,H 3 / V 

C « F - S0 ^ C1 ^t^ C 6 F 5 -/^C1 

cl 

Hg, 300° 
/— ► (C 8 F 5 ) 2 Hg(75%) 

X — ► (C,F 5 ) 2 S (70%) + (C,F 6 ) 2 S 2 (30%) 

Pentafluoroiodobenzene (and its chloro- and bromo-analogue) undergoes 
the Ullmann reaction to yield perfluorobiphenyl (72%) when heated with 

216 



Aromatic Fluorocarbons 

copper bronze ; this reaction may involve participation of the copper in a 
free-radical reaction : 

C,F 5 I + Cu(0) -^* Cu+ + I- + C„F 8 . -5*!l^ Q,F 5 .Q|F, 

or perhaps pentafluorophenylcopper is formed and Subsequently decomposes 
to give the biphenyl. 67 * Pentafluorophenylcopper , prepared from a penta- 
fluorophenylmagnesium halide and a cuprous halide, is known to yield 
perfluorobiphenyl when heated above 200° (c/. phenylcopper, which de- 
composes to biphenyl and copper at room temperature). 87b 

3. Reactions of Pentafluorophenyl Derivatives. In general, pentafluoro- 
phenyl derivatives containing the familiar functional groups of organic 
chemistry undergo the conventional, and in some cases classical, reactions 
associated with aromatic compounds bearing these groups (see the previous 
discussion and Figs. 5.1 and 5.2). Naturally the reactivities of functional 
groups and the stabilities of reaction intermediates are modified by the 
electronic properties of the pentafluorophenyl group (c/. p. 208). For ex- 
ample, pentafluoroaniline dissolves only in concentrated mineral acids and 
dilution of the resulting solutions with water liberates the free base ; 70 and 
acylation of the amino function proceeds sluggishly. 71 Pentafluoroaniline, 
being a weak base, also undergoes diazotisation only slowly when its 
solution in cold concentrated hydrochloric acid is treated with sodium 
nitrite, so almost complete conversion into decafluorodiazoaminobenzene 
occurs. 70,71 Diazotisation without immediate self-coupling can be accom- 
plished by a reverse-addition technique, but addition of the product to 
an alkaline solution of /J-naphthol yields azo compounds in which nuclear 
fluorine has been replaced by chloro or hydroxyl groups. The lability of 
nuclear fluorine in the pentafluorobenzenediazonium cation is also revealed 
by the following sequence, which establishes that fluorine situated para 
to the activating cationic group is displaced preferentially: 71 

1. 70% HjSOi, NaNO s / ^ / == V 

/ "H. I:S££» - HO-( )-X= N -/ )-N(CH 8 ) 2 

_' (acid conditions) x =2. N ' 

F F F 

Serious loss of ring fluorine can be prevented by diazotisation of penta- 
fluoroaniline in 80% hydrofluoric acid or anhydrous hydrogen fluoride 
(b.p. 19-5°) and use of these media enable, for example, successful con- 
version of the amino- compound into bromo- or iodo-pentafluorobenzene 
to be achieved via the classical diazonium route. 70,71 

Nucleophilic displacement of ring fluorine as fluoride is not restricted 
to pentafluorobenzenediazonium salts and can be made to occur, though 

15 217 




Perfluorinated Aromatic Compounds 

much less easily, with other pentafluorophenyl derivatives (see Table 5.3). 
Many reactions of the type where a pentafluorobenzene C 6 F 5 .X is converted 
into a tetrafluorobenzene Nu.C 6 F 4 .X by treatment with a source of a 
nucleophile Nu~ have been studied, and information regarding the effect 
of the nature of group X on both the ease and orientation of attack is 
available. Thus, kinetic studies 78 have established the following orders of 
reactivity towards (i) methoxide in methanol at 50° and (ii) methoxide in 
dioxan/methanol at 50° (rate ratios relative to C 6 F $ = 1 in parentheses 

Table 5.3. Examples' 2 of Tetrafluorobenzenes Formed by NueleopMUc Attack on 
Pentafluorophenyl Derivatives C„F 5 .X 



Pentafluorophenyl 


Nucleophilic 


Major product 


derivative 


reagent 


(> 90% unless stated) 


C.F..H 


NaOCH, 


p-CH 8 O.C,F 4 .H 


C,F 5 .H 


NaSH 


p-HS.C 6 F 4 .H 


C,F 6 .H 


KSC,H 6 


p-C g H 5 S.C,F 4 .H 


C,F S .H 


NH 8 


p-NHs.C.F^H 


C,F 5 .H 


N 2 H t 


p-NHj-NH-C^.H 


C 6 F 5 . SCH S 


NaOCH 3 


p-CH 8 O.C,F 4 .SCH 3 


C,F 5 .SCH 8 


NH 8 


p-NHj.CeF4.SCH3 


6 F 6 .SO 2 CH 3 


NHs 


p-NH a .C,F 4 .S0 1! CH s 


C.F..NH,, 


NH, 


TO-NH a .C (1 F 4 .NH 2 


CyBVNHj 


NA 


TO-NHji.NH.CjF4.NHa 


C,F B .N0 2 


NH 8 


0-NHj.C.Fj.NOj (70 % ) 
p-NHjj.CeF4.NOg (30 % ) 




NaOCH, 


p-CH3O.CeF4.NOij 


C,F 5 .CH 3 


LiCHg 


p-CH,.C,F 4 .CH3 


C,F 5 .CF S 


LiCHg 


p.CH s .C 6 F 4 .CF 8 


C,F S .CF 3 


LiAlH 4 


p-H.CeF4.CF3 



together with the main position of attack relative to X): (i) C 6 F 5 .CN 
(13-1 x 10*; para) > C 6 F 5 .CF 3 (12-3 x 10 s ; para) > C 6 F 6 Br (5-70; para) 
> C e F 5 H (0-288; para) > C 6 F 5 .OCH 3 (0-033; meta and para); (ii) C 6 F 6 Br 
(3-06; para)>C 6 F 6 I (1-83; para) > C 6 F 8 .G02 (0-485; para) > C 6 F B H 
(0-190; para) > C«F 5 .CH 3 (0-0141; para) > C 6 F s .OCH 3 (0-0129; meta and 
para) > C e F 5 .NH 2 (0-00102; meta) > C e F B .0- (3-86 x 10-«; meta). As in- 
dicated (see also Table 5.3), the fluorine substituent located para to group X 
is the main one to suffer nucleophilic displacement in the majority of 
pentafluorophenyl derivatives examined; only a few cases are known 
where meto-attack predominates (e.g., X = NH 2 , 74 O -76 ) or comparable 
amounts of meta- and para-fluorine replacement occur (e.g., X = OCH 3 , 78 
NHCH 3 74 ). The atypical amination (seeTable5.3) of pentafluoronitrobenzene, 
which is much more susceptible towards nucleophilic attack than hexa- 
fluorobenzene, is attributed to hydrogen-bonding between ammonia and 

218 



Aromatic Fluorocarbons 

the nitro group of the substrate. 74 Similarly, predominant displacement of 
bromine from bromopentafluorobenzene by cuprous salts in dimethyl- 
formamide, e.g., 

C.F,Br + CuCN «-***~«» , C .F,CN(82«/.) + CuBr 

is ascribed to formation of intermediate complexes in which solvated 
cuprous salts bond preferentially to the aryl halogen of lower electro- 
negativity.' 7 

Orientation and reactivity phenomena associated with nucleophilic sub- 
stitution of fluorine in pentafluorobenzenes (C 6 F S .X) have been discussed 
in terms of the interplay of inductive and resonance effects in such sys- 
tems 72 and of the stabilities of intermediate cr-complexes (C) believed to 
be formed, as in many S N Ar reactions, 78 in the rate-determining steps and 
to provide good models for the transition states involved: 79,80 



Nu F 
Nu-+CiF 6 '.X *=* ll ~ 




(Nu = nuoleophile) 
The latter approach is explained briefly below. 

ortto-substitution 



X 

1 




X 




X 


Nu A. 

F>( || F 


■*—*■ 


v> 


■«— > 


v> 


F^K^F 
F 




F 




F^- Jf 
F 


(D 1) 




(D2) 




(D3) 


mefa-substitution 










X 

F 




X 

i 




X 

l 


■*— *■ 


1 

Nu \l F 
F /\y 
F 


<— > 


1 

f^Nf 

Nu \l If 

F /x O^ 
F 


(El) 




(E2) 




(E3) 



15» 



219 



Perfluorinated Aromatic Compounds 
para-substitution 

X 




Ignoring conjugation effects (as arise, for example, when X = CN or 
N0 2 ), ^-complexes of type (C) are resonance hybrids of three canonical 
forms; and for any one hybrid it is assumed that the para- quinonoid 
structure is the major contributor. 79 Thus, for example, if the relative 
stabilities of the three possible a- complexes (D, E, and F; X = Br) in- 
volved in displacement of fluorine from bromopentafluorobenzene by a 
nucleophile Nu" (e.g., HO", CH 3 CT) are being assessed, attention is focussed 
on canonical forms (D2), (E2), and (F2). In addition, it is assumed 
(c/. p. 84) that in 7r-electron systems such as the a-complexes under con- 
sideration halogens destabilize a negative charge on adjacent carbon in 
the order C— F > C— CI > 5— Br > C— I (~C— H); this is the order for the 
+ I„-effect (F > CI > Br > I) — Coulombic repulsion between halogen lone- 
pair electrons and jr-electrons — indicated by u.v. spectroscopic data for 
the halogenobenzenes. 81 It follows, therefore, that meto-substitution of 
fluorine in bromopentafluorobenzene is least preferred since all three 
canonical forms contributing to the tr-complex contain a >C — F group, 
and that paro-fluorine should be displaced in preference to ortho-fbiovine 
owing to the stability order (F2) > (D2); in practice, 79 the compositions 
of the tetrafluorobenzenes (HO.C„F 4 Br and CH 3 O.C 6 F 4 Br) formed by re- 
action of bromopentafluorobenzene with potassium hydroxide and sodium 
methoxide are 15o:lm:Mp and 12o:lm:87p, respectively. This type of 
argument is readily extended to rationalize almost exclusive para-fluorine 
displacement in C 6 F 6 .X when X is capable of powerful electron withdrawal 
[e.g., NO a ( — I, — M), CF 3 (— I)] and to explain the incursion of meto-fluorine 
substitution when group X can exert a + I„-repulsive effect comparable with 
or greater than that of fluorine. B/elative reactivities of pentafluorophenyl 
derivatives in S N Ax reactions can be rationalised by combining assessments 
of transition state stabilities derived from inspection of ff-complex structures 
with data on substrate ground-state stabilities. 79 It has been pointed out 7 * 
that the structures of the di- and tri-chloro compounds formed in the re- 
action between hexachlorobenzene and potassium fluoride are in accord 
with the order given above for the I^-effect of the halogens. 

Finally, attention is drawn to the fact that pentafluorophenyl derivatives 
of many elements are known (Li, Cu, Mg, Zn, Cd, Hg, B, Al, Ga, In, Tl, 
Si, Ge, Sn, Pb, N, P, As, Sb, O, S, Se, Te, and transition metals), 82 in- 

220 



N -Heterocyclic Analogues of Hexafhtorobenzene 

eluding some which, as yet, have no perfluoroalkyl counterparts in terms 
of the element involved (e.g., derivatives of the Group III b elements Ga, 
In, and Tl) or of compound type [e.g., the bis-compound (CjFgJgMg; the 
tris-compound (C 6 F 6 ) 3 B ; the formally three-covalent compounds C 6 P 6 .AlBr 2 
and (C 6 F 8 )gAlBr]. The most important compounds from the synthetic 
viewpoint are pentafluorophenylmagnesium bromide and pentafluoro- 
phenyl-lithium, which are easily made and have been used extensively to 
prepare pentafluorophenyl derivatives of other elements from their halides 
(see p. 212 and ref. 82). In general, the differences in properties between 
the pentafluorophenyl derivatives and their hydrocarbon analogues are 
not as marked as with the perfluoroalkyl compounds. 



II. ^-HETEROCYCLIC ANALOGUES OF 
HEXAFLTJOROBENZENE 

At present pentafluoropyridine has received more attention than any 
other member of this class. It is a colourless, almost odourless liquid, b.p. 
83-3° (c/. pyridine, b.p. 115-3°; hexafluorobenzene, b.p. 80-5°), which was 
first prepared in 1959 by defluorination of perfluoro-JV-fluoropiperidine 
obtained by electrochemical fluorination of pyridine (see p. 12) r 83,84 

F 



2 (7%) ^,580-610°/! mm *jf ,- 

(see p. 133) pU H 



This highly inefficient method, which stemmed from work on the aroma tiza- 
tion of perfluorocyclohexadienes (see p. 204), was superseded in the early 
1960's by a halogen-exchange method [cf. p. 205) : 85,86 

CI 

^ PC1 5 , 350° (Nl autoclave) Cl(| ^|C1 anhydrous KP 500" 

' a4aa1 AY* + AAlA*TAflA 




.^" has also been developed. 8 ') 



r*xr uxm uunr hnrou ucvcivimu. — j \"\T- 



(A direct chlorination method »,[! U, steel autoclave" 



civ . Vci 



F 

F.r^iCi 

(83%) + f (7%) 

F^l JF Fl Jf 




which is well suited to commercial exploitation. Tetrafluoropyridazine, 89 
tetrafluoropyrimidine, 88 tetrafluoropyrazine, 90 and cyanuric fluoride 91 (and 
also heptafluoro-quinoline and -isoquinoline 92 ), the other known JW-hetero- 

221 



Perfluorinated Aromatic Compounds 

cyclic analogues of hexafluorobenzene, are also best prepared by fluorina- 
tion of the corresponding perchloro-cOmpounds with anhydrous potassium 
fluoride, e.g., 



CI 

PC1 " F0C S ^f^X (37%) KF ' 480 °> F | N (85 o /o) 
heat ~" 11 '"" -"* — 1 — — " '— 




CllJcl autoclave pO A F 



CI 
^\ TO*.**, £ ^ % 

ClU^ ,)CI * tt toclave F^ Jf 



Replacement of chlorine by fluorine in the perchloro-jV-heterocyclic com- 
pounds is achieved more easily than in hexachlorobenzene owing to the 
activating effect derived from electron-withdrawal by ring nitrogen in such 
systems (c/. the ease of hydrolysis of 4-chloropyridine with that of chloro- 
benzene) ; similarly, the perfluoro-^-heterocyclics produced are much more 
susceptible towards nucleophilic attack than is hexafluorobenzene, as dis- 
cussed below. 

Pentafluoropyridine, unlike pyridine, is not sufficiently basic to yield 
either a co-ordination compound with boron trifluoride 98 or a salt with 
hydrogen chloride; 84 thus pentafluoropyridine is a weaker base than penta- 
fluoroaniline (see p. 217), and the base-strength order alkylamines > pyridine 
> aniline does not apply in fluorocarbon chemistry [no data are available 
yet that allow pentafluoropyridine to be compared with perfluoroalkyl- 
amines (see p. 131) as regards lone-pair availability]. Like its hydrocarbon 
counterpart (c/. pyridine and benzene), pentafluoropyridine is considerably 
more susceptible to attack by nucleophilic reagents than its carbocyclic 
analogue, hexafluorobenzene. Thus, when an approximately equimolar 
mixture of pentafluoropyridine and hexafluorobenzene is treated with a 
deficiency of sodium methoxide in methanol, nucleophilic attack appears 
to occur exclusively on the pyridine since the only ether detected in the 
product is 2,3,5,6-tetrafluoro-4-methoxypyridine; 88 by means of a similar 
competition experiment, it can be shown that pentafluoropyridine shows 
essentially the same reactivity towards methoxide as pentafluoronitro- 
benzene.' 4 More qualitative evidence for the high reactivity of penta- 
fluoropyridine in S N Av reactions can be gleaned from preparative ex- 
perience; for example, 4-cyano- 9s and 4-iodo-tetrafluoropyridine 96 can be 
prepared directly from the pentafluoro-compound, which even reacts 
readily with weakly nucleophilic carbonylmetal anions such as [Mn(CO) 5 ]~ 97 
(see Fig. 5.3). It has proved a relatively simple matter, therefore, to produce 
a whole range of 4-substituted 2,3,5,6-tetrafluoropyridines, and the chem- 

222 



II 
jxi En 



o 






to 

■w 







223 



Perflvorinated Aromatic Compounds 

istry of these compounds is being developed along the lines pursued earlier 
with pentafluorophenyl derivatives. The 2,3,5,6-tetrafluoropyridyl group 
is more electronegative than the pentafluorophenyl moiety, as expected 
from the presence of the ring nitrogen atom and revealed by 8 N Ax reactions 
involving pentafluoropyridine and by the acid strength of tetrafluoro- 
4-hydroxypyridine [Z a (H 2 O,20°) = 7-7 x M)- 4 ; 98 cf. C 6 F 5 .OH, 3-0 x 10" 
at 25°], which, unlike 4-hydroxypyridine, exists predominantly as the 
pyridine and not the pyridone tautomer. 98,99 

As implied in the foregoing discussion, nucleophilic attack on penta- 
fluoropyridine initially occurs almost exlusively at the 4-position (the order 
of ease of displacement of ring fluorines decreases in the order 4- > 2- or 
6- » 3- or S-). 98 ' 98 This can be interpreted in terms of the relative stabilities 
of the transition states involved in the rate determining steps for sub- 
stitution at the different positions, using the cr- complexes as models for 
these states, as described previously when the orientation of nucleophilic 
attack on pentafluorophenyl derivatives was under discussion (see p. 219). 
The intermediates (G), (H), and (J) arising from attack at the 2- (or 6-), 
3- (or 5-), and 4-positions, respectively, in pentafluoropyridine are resonance 
hybrids, each with three contributing forms (1, 2, and 3). Forms in which 
the negative charge resides on the carbon of a > CF group are destabilized 
by the I„-repulsive efFect of fluorine and are presumably much less stable 
than those in which a ring nitrogen atom carries the charge. Thus attack 
by a nucleophile at the 3-(or 5-)position is least preferred. The greater 
reactivity of the 4-position compared with that of the 2-(or 6-)position 
reflects the higher stability of hybrid (J) compared with (G), presumably 
because the para-quinonoid forms (J2) and (G2) (the former is the more 
stable since the negative charge resides on nitrogen) contribute more to 
the structures of the a-complexes than do the orifeo-quinonoid forms [(Jl 
and 3) and (Gl and 3)]. Efficient accommodation of the negative charge 
of an attacking nucleophile by the ring nitrogen in pentafluoropyridine is 
also reflected in the reactivity order C 6 F 5 N > C 6 F 6 . 



2- Substitution 






F 

F|^^>,F 

J !/ F 


■*— > 


F F 
Fi^^F F(j' / 'Nf 

J P ~J V s 


^N/\ Nu 




F ^N/\ Nu F ^N/\ Nu 


(Gl) 




(G2) (G3) 


3- Substitution 






F F 

x Nu 

fI Jf 


■*— *■ 


F F F F 

T is T is 

x Nu *-*• N Nu 

Fk- Js fL ->'f 



(HI) (H2) (H3) 

224 



N -Heterocyclic Analogues of Hexafluorobenzene 



4- Substitution 



F Nu 
/ 

(Jl) 



1 

pll 



F Nu 
(J2) 




(J3) 



As with pentafluoropyridine, nucleophilic attack on tetrafluoropyrid- 
azine 89 and on tetrafluoropyrimidine 88 leads to preferential displacement 
of fluorine from position 4 (i.e., para to a ring nitrogen), e.g., 



F 

FtT "**|F 

N 
N^ 



Fi 



F 

N 



Fl JF 



NH, 



cone. NH t aq. 



F, 



>F 

N 

O.CH 3 



CH..OH, Ka>CO» 



20" 



F Vx> 



and the ease of attack is greater in each case than with tetrafluoropyrazine, 90 
in which all four fluorines lie ortho to ring nitrogen, or with pentafluoro- 
pyridine, which contains only one nitrogen atom. These facts can also be 
rationalized by consideration of the relative stabilities of intermediate 
ff-complexes used as models for transition states. 88 Cyanuric fluoride, as 
expected, is extremely susceptible towards attack by nucleophilic rea- 
gents and yields cyanuric acid rapidly when treated with an excess of 
water at ca. 0°. M 

No general method of preparation of 2-substituted tetrafluoropyridines 
has been developed yet; compounds containing highly electronegative 
2-substituents can be synthesised from perfluorocyclohexa-l,3-diene and 
the appropriate nitriles, e.g., 100 



F 

F r i F 2 400 

+BrCN' *" 



r f 



k>. 




[Br 



n 



-C,F 4 F 

>■ 

F 




(40%) 



N^ N Br 



3- Substituted compounds can be prepared from 3-chlorotetrafluoropyridine, 
which is prepared by fluorination of pentachloropyridine with potassium 



225 



Perfluorinated Aromatic Compounds 
fluoride under controlled conditions, e.g., wl 

F F 

Fi|NGl H a ,Pd/C FfNH a/x n-CH.Ll, hexane 

11 ' " ' (co. 75%) — > 



F F 






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3. Babbottb and Thomas, Ind. Eng. Chem., 1966, 58, 48. 

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226 



References 

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29. Brooke, Bubdon, and Tatlow, Chem. <b Ind., 1961, 832. 

30. Brooke, Bubdon, and Tatlow, J. Chem. Soc, 1961, 802. 

31. Forbes, Richardson, and Tatlow, Chem. <k Ind., 1958, 630. 

32. Birchall, Haszeldine, and Parkinson, J. Chem. Soc, 1962, 4966. 

33. Forbes, Bichardson, Stacby, and Tatlow, J. Chem. Soc, 1959, 2019. 

34. Robson, Staoey, Stephens, and Tatlow, J. Chem. Soc, 1960, 4754. 

35. Birchall and Haszeldine, J. Chem. Soc, 1961, 3719. 

36. Barbour, Buxton, Coe, Stephens, and Tatlow, J. Chem. Soc, 1961, 808. 

37. Birchall, Clarke, and Haszeldine, J. Chem. Soc, 1962, 4977. 

38. Birchall and Haszeldine, J. Chem. Soc, 1959, 3653. 

39. Ingold, Structure and Mechanism in Organic Chemistry, Bell and Sons, London, 
1953, p. 75; Gould, Mechanism and Structure in Organic Chemistry, Holt, 
Rinehart, and Winston, New York, 1959, p. 212. 

40. Massey, Randall, and Shaw, Ohem. <fc Ind., 1963, 1244; Holmes, Peacock, 
and Tatlow, Proc Chem. Soc, Lond., 1963, 108; J. Chem. Soc (A), 1966, 
150. 

41. Filler and Wang, Chem. Comm., 1968, 287. 

42. Bbuce and Stone, J. Chem. Soc. (A), 1966, 1837. 

43. Burdon, Damodaran, and Tatlow, J. Chem. Soc, 1964, 763. 

44. Cob, Jukes, and Tatlow, J. Ohem. Soc. (C), 1966, 2323. 

45. Beckert and Lowe, J. Org. Chem., 1967, 82, 582. 

46. Brooke, Chambers, Heyes, and Musgrave, J. Chem. Soc, 1964, 729. 

47. Bbewek, Eckhabd, Heaney, and Mabples, J. Chem. Soc (C), 1968, 664. 

48. Noltes and van den Hurk, J. Organometal. Chem., 1964, 1, 377. 

49. Deacon, Gbeen, and Nyholm, J. Chem. Soc, 1965, 3411. 

50. Rausch, Inorg. Chem., 1964, 3, 300. 

51. Birchall, Bowden, Haszeldine, and Lever, J. Chem. Soc. (A), 1967, 747. 

52. Sohmeisser and Weidenbruch, Chem. Ber., 1967, 100, 2306. 

53. Cohen, Reddy, and Massey, J. Organometal. Chem., 1968, 11, 563. 

54. Hopton, Rest, Rosevear, and Stone, J. Chem. Soc. (A), 1966, 1326. 

55. Coe, Stephens, and Tatlow, J. Chem. Soc, 1962, 3227. 

56. Harper, Soloski, and Tamborski, J. Org. Chem., 1964, 29, 2386. 

57. Streitwieser, Hudson, and Mares, J. Amer. Chem. Soc, 1968, 80, 648. 

58. See Cohen, Tomlinson, Wiles, and Massey, J. Organometal. Chem., 1968, 
11, 385 and refs. quoted therein. 

59. Callander, Cob, and Tatlow, Chem. Comm., 1966, 143; Callendeb, Coe, 
Tatlow, and TJfp, Tetrahedron, 1969, 25, 25. 

60. Fields and Meyebson, J. Org. Chem., 1967, 32, 3114. 

61. Claret, Williams, and Coulson, J. Chem. Soc. (C), 1968, 341. 

62. Charles, Pearson, and Whittle, Trans. Faraday Soc, 1963, 59, 1156; Bir- 
chall, Davison, and Haszeldine, unpublished results. 

63. Birchall, Daniewski, Haszeldine, and Holden, J. Chem. Soc, 1965, 6702. 

64. Godsell, Stacey, and Tatlow, Nature, 1956, 178, 199. 

227 



Perfluorinated Aromatic Compounds 

65. Bibohall, Haszeldine, and Pabkinson, J. Chem. Soc, 1962, 4966. 

66. Bain, Blackman, Cummings, Hughes, Lynch, McCall, and Robebts, Proc. 
Chem. Soc, Lond., 1962, 186. 

67. (a) Bibohall, Hazabd, Haszeldine, and Wakalski, J. Chem. Soc. (G), 1967, 

47. 
(b) Catbnoboss and Sheppabd, J. Amer. Chem. Soc, 1968, 90, 2186. 

68. Beckebt and Lowe, J. Org. Chem., 1967, 82, 1215. 

69. Cohen, Reddy, and Massey, Chem. Comm., 1967, 451. 

70. Wall, Pummeb, Feabn, and Antonucci, J. Res. Nat. Bur. Stand., 1963, 
67 A, 481. 

71. Bbooke, Fobbes, Richardson, Stacey, and Tatlow, J. Chem. Soc, 1965, 
2088. 

72. For a review of nueleophilic substitution in aromatic fluorocarbons see Tatlow, 
Endeavour, 1963, 22, 89. 

73. Ho and Miller, Austral. J. Chem., 1966, 19, 423. 

74. Allen, Bubdon, and Tatlow, J. Chem. Soc, 1965, 6329. 

75. Bubdon, Hollyhead, and Tatlow, J. Chem. Soc, 1965, 5162. 

76. Allen, Bubdon, and Tatlow, J. Chem. Soc, 1965, 1045. 

77. Belf, Buxton, and Fulleb, J. Chem. Soc, 1965, 3372. 

78. See, for example, Bunnett and Gabst, J. Amer. Chem. Soc, 1965, 87, 3879, 
and references quoted therein. 

79. Bubdon, Tetrahedron, 1965, 21, 3373; Bubdon, Coe, Marsh, and Tatlow, 
ibid., 1966, 22, 1183. 

80. Bibohall, Gbeen, Haszeldine, and Pitts, Chem. Comm., 1967, 338. • 

81. Clark, Mubbell, and Teddeb, J. Chem. Soc, 1963, 1250. 

82. For a review of pentafluorophenyl derivatives of Zn, Hg, B, Al, Ga, In, Tl, 
Si, Ge, Sn, Pb, P, As, Sb, and transition metals see Chambebs and Chivebs, 
Organometal. Chem. Rev., 1966, 1, 279. Consult also Caincboss and Sheppabd, 
J. Amer. Chem. Soc, 1968, 90, 2186, and Dua, Jukes, and Gilman, J. Organo- 
metal. Chem., 1968, 12, P24, P44 (Cu); Respess and Tambobski, J. Organo- 
metal. Chem., 1968, 11, 619 (Mg); Schmeisseb and Weidenbruch, Chem. Ber. 

1967, 100, 2306 (Cd); Connett, Davies, Deacon, and Gbeen, J. Chem. 
Soc (C), 1966, 106 and Deacon, J. Organometal. Chem., 1967, 9, PI (Hg); 
Dickson and West, Austral. J. Chem., 1966, 19, 2073 and Chambebs and 
Cunningham, J. Chem. Soc. (C), 1967, 2186 (Al); Feabon and Gilman, J. 
Organometal. Chem., 1967, 10, 409 and Lappebt and Lynch, Chem. Comm., 

1968, 750 (Si); Fenton, Massey, and Ubch, J. Organometal. Chem., 1966, 6, 
352 (Ge); Hills and Henby, J. Organometal. Chem., 1967, 9, 180 (Pb); Gbeen 
and Ktbkpatbick, J. Chem. Soc. (A), 1968, 483 (As); Cohen, Reddy, and 
Massey, J. Organometal. Chem., 1968, 11, 563 (S, Se, and Te); and Chubchill, 
O'Bbien, Rausch, and Chang, Chem. Comm., 1967, 992, Chaudhabi and 
Stone, J. Chem. Soc. (A), 1966, 838, and Cohen, Fenton, Tomlinson, and 
Massey, J. Organometal. Chem., 1966, 6, 301 (transition metals). 

83. Banks, Ginsberg, and Haszeldine, Proc Chem. Soc, Lond., 1960, 211; J. 
Chem. Soc, 1961, 1740. 

84. Bubdon, Gilman, Patbick, Stacey, and Tatlow, Nature, 1960, 186, 232. 
88. Banks, Haszeldine, Latham, and Young, J. Chem. Soc, 1965, 594. 

86. Chambebs, Hutchinson, and Musgbave, J. Chem. Soc, 1964, 3573. 

87. B.P. 1,041,906/1966. 

88. Banks, Field, and Haszeldine, J. Chem. Soc. (C), 1967, 1822. 

89. Chambebs, MaoBride, and Musgbave, Chem. & Ind., 1966, 904; J. Chem. 
Soc. (G), 1968, 2116. 

90. Chambebs, MacBbide, and Musgbave, Chem. & Ind., 1966, 1271. 

228 



References 

91. Dorlars, G.P. 1,044,091/1988. See also Maxweix, Fby, and Bigelow, J. 
Amer. Chem. Soc., 1958, 80, 548; Gbisley, Gluhsenkamp, and Heininger, 
J. Org. Chem., 1958, 28, 1802; and Tuijoook and Coffman, ibid., 1960, 25, 
2016. 

92. Chambebs, Hoik, Iddon, Musgrave, and Storey, J. CAew. Soc. (G), 1966, 
2328. 

93. Chambers, Hutchinson, and Musgrave, J. Chem. Soc., 1964, 3736. 

94. Chambers, Hutchinson-, and Musgrave, J. Chem. Soc. (C), 1966, 220. 

95. Banks, Haszeumne, and Yottng, J. Chem. Soc. (C), 1967, 2089. 

96. Banes, Haszeldine, Phhxips, and Yottng, J. Chem. Soc. (C), 1967, 2091. 

97. Booth, Haszei/dine, and Taylor, J. Organometal. Chem., 1966, 6, 570; 
Cooke, Green, and Stone, J. Chem. Soc. (A), 1968, 173. 

98. Banks, Burgess, Cheng, and Haszeldine, J. Chem. Soc., 1965, 575. 

99. Chambers, Hutchinson, and Musgrave, J. Chem. Soc, 1964, 5634. 

100. Anderson, Feast, and Musgrave, Chem. Comm., 1968, 1433. 

101. Chambers, Dbakesmith, and Musgrave, J. Chem. Soc, 1965, 5045. 



r 



229 



APPENDIX 

Fluorocarbon chemists make extensive use of infrared and nuclear 
magnetic resonance spectroscopy to obtain structural information, and the 
object of this section is to present introductory remarks concerning the 
identification of polyfluorinated organic compounds by these methods. 
Relatively little use is made of ultraviolet and mass spectrometry, although 
it should be noted that not much detailed attention has yet been paid to 
the analysis of organofluorine compounds by the latter method. 1 



I. IKFBAEED SPECTROSCOPY 

Infrared absorption spectra of fluorocarbons and related compounds are 
normally obtained with standard spectrophotometers operated in the 
ordinary i.r. region [4000-667 cm -1 (2-5-15 fz)]. Standard sampling tech- 
niques are employed, gas sampling being the most popular owing to the 
characteristic high volatilities of the compounds. 

A fairly recent comprehensive review of the vibrational spectra of organic 
fluorine compounds is available. 8 In general, the i.r. spectra of fluorocarbons 
and related compounds show a highly intense and often extremely com- 
plex absorption band system (which commonly defies analysis) in the 
1400-1000 cm -1 (7-1-10.0 jx) range that is associated mainly with C — F 
stretching modes. The spectrum of a saturated fluorocarbon is thus very 
characteristic since only weak absorptions appear in the 4000-1400 cm -1 
(2-5-7-1 (i.) range, those in the region of 2600-2100 cm" 1 (3-84-4-76 (x) being 
caused by weak overtones of the very strong C — F stretching fundamentals. 
Fortunately, then, the strong absorptions associated with C — F stretching 
vibrations do not encroach on the 'double and triple bond' region ; thus the 
spectra of unsaturated fluorocarbons, aromatics such as hexafluorobenzene 
and pentafluoropyridine, and fluorocarbon derivatives containing multiple- 
bonded functional groups clearly show characteristic absorptions in the 
2500-1400 cm -1 (4-0-7-1 [i) range (see Fig. 1), but usually at higher fre- 
quencies than their hydrocarbon analogues [e.g., cf. CF 3 .CF 2 .CF:CF 2 , 
v(C=C str.) 1792 cm- 1 , and CH 3 .CH 2 .CH:CH 2 , v(C=C str.) 1645 cm- 1 ]. 
Hydrogen- containing groups, such as 7C — H, >N— H, O — H, and S — H, 
can be detected by the presence of the usual stretching vibrations near 

3000 cm- 1 (3-35 (i.) [e.g., (CF 3 ) 2 CF— H, 2990 cm- 1 (3-34 n) ; CF2.fCFJ4.N-H, 
3448 cm- 1 (2-90 (A,; the N— H deformation band occurs at 6-75 ,u) ; CF 3 .S— H, 

230 



™F Nuclear Magnetic Resonance Spectroscopy 

2618 cm- 1 (3-82 ji)]. The review mentioned above should be consulted for 
a detailed discussion of these points and of many other structure-absorption 
correlations. 

II. "FNUCLEAE MAGNETIC RESONANCE 
SPECTROSCOPY 

High-resolution nuclear magnetic resonance (n.m.r.) spectroscopy is a 
more powerful tool than infrared spectroscopy for determining the structures 
of polyfluorinated compounds. The fluorine nucleus is an ideal subject for 
n.m.r. investigation : the element is univalent and only one isotope occurs 
naturally— M F, which has a spin number (/) of 1/2, and thus does not 
possess a quadrupole moment, and a gyromagnetic ratio similar in mag- 
nitude to that of the *H nucleus. Although the fluorine nucleus is less 
sensitive to detection by n.m.r. techniques than the protium and easier 
to saturate, 19 F spectra are easily obtained using standard commercial 
spectrometers; the resonance frequency of the fluorine nucleus at 14,090 
gauss is 5646 MHz (c/. r H = 60-00 MHz) and its relative sensitivity is 
0-834 ^H = 1-000). Information concerning experimental techniques, 
analysis of spectra, and theoretical interpretation of the origins of 19 F 
chemical shifts and coupling constants can be found in standard texts; 3 
trends in the values of fluorine chemical shifts and coupling constants are 
surveyed in the following discussion, which was written by Dr. M. G. Barlow 
and includes a unique chart of M F chemical shifts (Fig. 2) assembled from 
data published in the literature or extracted from spectra recorded in the 
Chemistry Department of the University of Manchester Institute of Science 
and Technology. It is quite obvious that fluorine chemical shifts and coup- 
ling constants cover much larger ranges of values than found in l H spectra, 
and 'fall off' in values of coupling constants as the number of intervening 
bonds increases is much less pronounced in the case of fluorine spectra; 
in fact, many cases are known where couplings between vicinal fluorines 
are .weaker than those between more distant nuclei. Owing to the larger 
internal chemical shifts, interpretative complications arising from overlap 
of band systems are encountered less frequently with 19 F than with X H 
spectra; however, since 19 F coupling is observable over larger internuclear 
distances, multiplet structure of band systems is frequently more compli- 
cated. 

No completely satisfactory reference compound has been proposed for 
19 F chemical shifts, and the values quoted below refer to those commonly 
employed, viz., trifluoroacetic acid (external reference) and trichlorofluoro- 
methane (internal reference). Tiers, who introduced the quantity t = 10 - 8 
in *H n.m.r. spectroscopy, advocates that 19 F chemical shifts should be 
reported at infinite dilution in trichlorofluoromethane according to the 
0-scale, where <Z>*(p.p.m.) = 10« (H mm ^ - ffcFci,)/#circi, and =#* at 

231 



Appendix 

infinite dilution; 4 solutions of < 10% in CFC1 3 can be regarded as infinitely 
dilute and extrapolation to infinite dilution can be conducted when it is 
impractical to examine dilute solutions. Conversion of chemical shifts to a 
common scale is strictly not permitted owing to the occurrence of solvent 
effects, but in practice, and to a fair approximation, shifts (p.p.m.) relative 
to CF 3 -C0 2 H may be converted to the 0-scale by the addition of 78. 



Fluorine Chemical Shifts and Coupling Constants 

Saturated Aliphatic Compounds 

No simple correlation exists between chemical shifts for fluorine nuclei 
and the electronegativities of either directly-bonded substituents or groups 
bonded to an intervening carbon atom; however, the observed shifts, for 
example, of the fluorine nuclei in OF, NF 2 , and CF S groups fall in the 
expected order, and, in general, the shifts for CF 3 , > CF 2 , and 7CF groups 
carrying one common substituent occur increasingly to high field. For 
fluorines bonded to carbon, the largest values of chemical shift for any one 
type of environment are usually observed in the perfluorinated compounds, 
and replacement of fluorine on adjacent carbons by almost any other sub- 
stituent, e.g., a fluoroalkyl group, hydrogen, chlorine, causes a low-field 
shift. The shifts of fluorines in CF 2 groups in alicyclic compounds are 
dependent upon ring size, their chemical shifts (relative to CF 8 -C0 2 H) in 
perfluorocyclo-propane, -butane, -pentane, and -hexane being 82-4, 58-6, 
56-9, and 57-0 p.p.m. (average of axial and equatorial fluorines), respectively ; 
and again substituents other than fluorine on adjacent carbons usually 
cause a low-field shift, the effect being greatest for the fluorine ci8 to the 
substituent. 

In Table 1 are shown ranges of magnitude for a variety of F— F and 
H — F coupling constants. In saturated organic fluorine compounds geminal 
F — F couplings cover rather a large range of magnitudes, and in acyclic 
compounds of the type CF 2 Q-CHFC1, where a range of 142-343 Hz has 
been reported, the magnitude is inversely related to the electronegativity 
of the substituent Q. 6 Substituents on an adjacent carbon have a much 
less pronounced effect upon the geminal coupling. Replacement of fluorine 
by hydrogen, chlorine, bromine, or iodine reduces the observed coupling 
by ca. 5, 15, 20, and 15 Hz, respectively; while a perfluoroalkyl substituent 
leads to values larger by some 2 Hz. 8 In alicyclic compounds, the magnitude 
is dependent upon ring size, observed ranges for cyclo-propanes, -butanes, 
-pentanes, and -hexanes being 156-212, 195-240, 251-276, and 276-305 Hz, 
respectively, 9 and the largest values are associated with completely fluori- 
nated derivatives. 

The vicinal F — F couplings in acyclic compounds cover a much smaller 
range of magnitudes. The smallest values are associated with coupling within 

232 



**F Nuclear Magnetic Resonance Spectroscopy 
Table 1. Magnitudes of F— F and H— F Coupling Constanta (Hz) 



\ / F F \ / F F \ / F F \ / F 

yc<^ x c— <r x c— c— ck x c— c— c— <r 

43-2-372 0-391 65-13-6 0-180 

\ / H H \ / F H \ / F H \ / F 

)>c(^ x c— c x N c— c— c x x c— c— c— c' 

41-5-79-7 1-2-29 0-1-6 0-0-7 

C=C< X C=C X 
X F 

0-112 0-58 

y H H\ y V 

69-1-89-9 0-22 



F 
«^Xt 



18-5-35-2 



H 




7-4-11-8 






perfluoroalkyl groups— the range 0-4-6 Hz has been reported for penta- 
fluoroethyl groups — and larger values are found where substituents on the 
two carbon atoms involved are less electronegative than fluorine. 7 ' 8 A more 
limited range still is associated with long-range couplings, although coupling 
through five-bonds is often quite appreciable and may be of larger magnitude 
than shorter-range couplings. Six-bond couplings, through five carbon 
atoms, are usually small or negligible. 

The geminal H— F coupling is usually characteristic, and the range quoted 
in Table 1 may be further divided : in CHF 2 groups a range of 51-3-57-5 Hz 
and in CHF groups a range of 41-5-56-5 Hz has been observed. Larger 
values are observed for CHF groups in cyclopropanes 6 (55-6-63-5 Hz) and 

16 233 



Appendix 

are at a maximum in fluoroform. 9 Vicinal H — F couplings are rather variable 
and conformationally dependent, 10 while longer range H — F couplings in 
saturated acyclic compounds are usually small. 

Oleflnic Compounds 

Chemical shifts of olefinic fluorines in a variety of environments are given 
in Fig. 2; and those of olefinic fluorines in a variety of trifluorovinyl com- 
pounds, CF 8 :CFZ, 6,11_13 are listed in Table 2. Replacement of fluorine by 
a substituent Z causes a shift of the geminal fluorine to high-field, while 
the fluorines cis and trans to the substituent move to low-field [the sub- 



Table 2. Olefinic Fluorine Chemical Shifts in Trifluorovinyl Compounds 
(*)F N 
(c)F' 



(t)F. J?(g) 

>C=C< 



Substituent 


Chemical shifts 


(p.p.m.) rel. 


to CF $ -C0 8 H 


Z 


F 


F. 


F t 


F 


57-7 


57-7 


57-7 


H 


108 


50 


26 


CI 


68 


44 


28 


Br 


67-7 


40-6 


20-5 


I 


71-8 


35-5 


10-3 


OCH„ 


63-9 


56-9 


60-7 


OCF, 


62-8 


48-5 


40-8 


SCF„ 


76-5 


26-6 


6-9 


SO,F 


103-2 


13-0 


2-1 


NCCF.Jj, 


66-9 


33-2 


193 


As(CF:CF 2 ) 2 


99-2 


44-9 


7-0 


COF 


111-8 


13-8 


1-1 


CN 


114 


26-6 


3-2 


CF, 


119 


32-3 


19-0 


Si(CH,), 


122 


40-2 


113 


Ge(CH,), 


119 


42-0 


10-0 


Sn(CH,) 8 


118 


44-6 


9-3 


BClj 


106-7 


10-1 


-6-2 


HgtCF^Fj) 


107-2 


46-7 


12-1 


FefCO^-CjH,) 


69-3 


61-6 


11-5 


Re(CO) 5 


76-3 


53-7 


18-1 



stituent Fe(CO) a (7r-C 6 H 6 ) is an exception in this respect], the eflFect being 
greatest for the trows-fluorine. This also tends to be true of disubstituted 
compounds, introduction of a second substituent causing a further low- 
field shift of fluorines cis or trans to it, although the substituent effects are 
only approximately independent. 6 For olefinic F — F coupling constants 
(see Table 1), it is generally true that Jraras- couplings are of larger magnitude 

234 



2500 



2000 



cm' 



CtyC s=CF 
R F -N=C=0 
R F -C=N 
fCF 3 ) 2 C = C=0 
« F ) 2 C=N 2 
(R F orF] 2 C=C=CF 2 

Rf-CF=0 

RfC=0 
R F -CfOH)=0 

(R F ) 2 C=0 
R F -C(0R)=fl 
R F -CH = 
R F -C(NH 2 )=0 

R F -N-GF 2 
R F -N=CF-R F 
R F -CF-CF 2 
fCF 3 ) 2 C=CF 2 

M-CF^CF 2 
CF 2 = CF-CF=CF 2 
xCF=CF~. 

F V F 

R f -N=0 

R F -N0 2 

(Rp-perfluoroalkyl ; R - alkyl; 
M-Bi,Si,Ge l Sr),Asot)ty 



J L 



J L 



n» 1 



' ' ' 



I I 



2 3 k 



1500 
i 



i I l L 



I I 




' I I L 



WjQVelength (Microns) 

Fia. 1. Characteristic group frequencies (stretching vibrations) for some common multiple -bonded fluorocarbon compounds. 



a F Nuclear Magnetic Resonance Spectroscopy 

than cis-couplings, and in disubstituted olefins the effect of the substituents 
upon these is additive and independent. 14 The range of coupling constant 
magnitudes for gremiwaJ-fluorines overlaps that for cts-fluorines and they 
may be confused. However, an approximately linear relationship exists 
between the size of geminal F — F coupling and the mean of the chemical 
shifts of the nuclei involved 15 and this may aid structural identification. 

For olefinic H— F couplings, the geminal coupling, which is least in 
<ra«s-l,2,3,3,3-pentafluoropropene 6 and at a maximum in 1-fluoropropene, 18 
is characteristically large; and, like F— F couplings, trans- are of larger mag- 
nitude than cis-H—F couplings. There is some overlap of the ranges ob- 
served for the latter two couplings (see Table 1), but it is generally true 
that for any cisjtrans pair of isomeric olefins, or for olefins of the type 
CF 2 :CHZ, the cis- is of smaller magnitude than the *rare*-coupling, and 
the smallest values are associated with the more electronegative olefinic 
substituents. 

Ranges of magnitudes of couplings of olefinic protons and fluorine-nuclei 
to fluorines in side-chain substituents are rather dependent on the nature 
of the side-chain. In Table 3 are shown ranges observed for coupling to 
fluorines of trifluoromethyl side-chains; coupling of the CF 3 fluorines to 
cts-fluorines is generally greater than that to *raws-fluorines, and cis- 
couplings to olefinic protons are usually greater than <ran*-couplings also. 



Table 3. Magnitudes of F— F and H— F Couplings in 3,3,3-Trlfluoropropenes (Hz) 



/CF, 
C=C< 
N F 


X C= 


/CF S 
=C X 


X C=C X 

X CF 8 


7-3-13-7 


17-2 


-25-1 


5-8-13-4 


/CF 3 
C=C<( 


H \ 
N C= 


/CFs 
=C X 


H \ 

N C=C X 

X CF S 


6-3-7-8 


0-6-2-2 


0-0-9 



Aromatic Compounds 

The ranges of chemical shifts observed for fluorines ortho-, meta-, and 
para- to the substituent X in a pentafluorophenyl compound, C„F 5 X, are 
shown in Figure 2, and the shifts (converted to a hexafluorobenzene refer- 
ence—negative values are to low-field) for a variety of such compounds are 
shown in Table 4. 8 - 17 The ranges of chemical shiftB are greatest for otiho- 
fluorines and least for meto-fluorines. The or<Ao-shifts are rather scattered, 
and tend to increase down any one group of the Periodic Table, while 
meta- and para-shifts have been correlated with inductive and resonance 

16 * 235 



Appendix 

Table 4. Aromatic Fluorine Chemical Shifts in Fentafluorophenyl Compounds, C,F 5 X 



Substituent X 


Chemical shifts (p.p.m.) rel. to C„F 6 




Fortho 


-^ meta 


F 

•*■ para 


H 


-23-6 


-0-5 


-91 


CI 


-22-1 


-1-5 


-6-6 


Br 


-300 


-2-0 


-8-0 


I 


-43-4 


— 2-9 


-100 


CH S 


-18-6 


+ 1-2 


-3-5 


CF 8 


-22-8 


-2-6 


-150 


CgH 5 


-18-8 


+ 0-1 


-6-2 


CN 


-30-2 


-3-5 


-19-2 


COjjH 


-25-3 


-1-3 


-15-3 


CH:CH 2 


-18-0 


+ 1-5 


-5-5 


N0 2 


-15-6 


-30 


-140 


NH 2 


+ 0-9 


+ 2-7 


+ 10-9 


N(CH 3 ) 2 
OH 


-9-7 
+ 11 


+ 3-8 
+ 2-6 


+ 4-8 
+ 7-9 


OCH s 


-4-3 


+ 1-9 


+ 2-5 


Si(CH 3 ) 3 
Sn(CH 3 ) 3 

Pb(C 6 H 5 ) 3 

pm 


-38-2 -0-8 
-40-0 ■— 1-6 
- 45-2 - 2-7 
-33-9 to -24-5 —2-1 to +1-2 


- 10-6 
-9-5 
-9-7 

- 16-4 to - 5-8 


AsCla 

SCHj 

BQL, 

Al(Br 2 ) 2 

HgC,F 6 

HgCH 3 


-32-8 

-26-5 

-33-1 

-41 

-44-6 

-40-4 


-31 
+ 2-1 
-1-3 
-2-8 
-2-6 
-2-2 


-16-6 

-51 

-151 

-151 

-9-2 

-8-8 



parameters. 18 In di- and tri-substituted polyfluorobenzenes, the effects 
of the substituents upon the fluorine chemical shifts are approximately 
additive and independent, 19 and their shifts may be predicted from the 
shift data for the: appropriate pentafluorobenzenes in Table 4, appropriate 
note being taken of the position of the substituents, ortho, meta, or para, 
with respect to the fluorine under consideration. 

The observed ranges of magnitude for aromatic F— F and H— F couplings 
in fluorobenzenes are shown in Table 1. The trends of H— - F couplings 
mirror the corresponding trends of H— H couplings, except for some overlap 
of the ortho- and meta-ranges, but the trends of F— F couplings are much 
less clear cut. Ortho F— F couplings are usually fairly distinctive, the values 
at the upper end of the range being observed only in compounds with a 
transition-metal substituent ortho to one of the fluorines involved. 20 The 
magnitude of a weto-F— F coupling in particular is very dependent upon 
the nature and position of the remaining ring substituents, and an additivity 
relationship has been proposed to correlate the observed values. 21 

236 



•160 
_J 



-120 



■80 



OF 

SF 4 F 

S0 2 F 

NF 8 

COF 

N.COF 

N:CF lf2 

CF 3 I 

CF 3 Br 

CF,C1 



CF 3 

CF 3 As 

CF 8 P 

CF 3 N 

CF.I 



CF 3 C:C 

CFX 2 

CF 4 Se 

CF 8 S 

CF a O 

CFXN, CF 2 N 

CF a P 

CFX 

CFjj 

CFjjSi 

CF a Sn 

CF S H 

CFXH 

CFM 

CFO 

CF8 

CFN 

CF 

CFH 

CFH, 

CF 2 : C 
CF 2 : OF 
cis-CF:CF . 
trans-CF:CF- 
CF:C 



F 



m 
P" 



•40 



CFCl 3 



40 

I 



•240 



•200 



-160 



-120 



T> 



80 
I 



120 



160 

1_ 



200 



240 
j 



p. p.m. 



280 
I 



CF 3 -C0 2 H 



40 

transition metal) 



80 



120 



160 



p. p.m. 



200 



Fig. 2. 19 F Chemical shifts (X = Moj r en other than F; M 
Group A: Where the fluorine nuclei under consideration are attached to a carbon aton; i (e.g., CF 2 , CFXH, CFO,N:CF 2 )or to a hetero-atom (e. g., OF, K* 2 
substituent is ^-hybridized carbori . where all the co-substituents are shown (e. g., ( !F 3 S, CF 3 • C, CF 3 • C:C),the other groups attached may cover a ^ 

Group B: The unstated subs (bituents vary widely in type. 



SO a F), any unstated 
wide variety of types. 



REFERENCES 



1. Game, Tetrahedron, 1968, 24, 1811, and references quoted therein. 

2. Brown and Morgan, in Advances in Fluorine Chemistry, ed. Stacey, Tatlow, 
and Sharpe, Butterworths, London, 1965, Vol. 4, p. 253. (Also see Treichel 
and Stone, in Advances in Onganometallic Chemistry, ed. Stone and West, 
Academic Press, New York, 1964, Vol. 1, p. 212, for a discussion of the i.r. 
spectra of some fluorocarbon-metal compounds.) 

3. (a) Emsley, Feeney, and Sutcliffe, High Resolution Nuclear Magnetic 

Resonance Spectroscopy, Pergamon Press, Oxford, 1965, Vols. 1 and 2. 
(Chapter 11, Vol. 2, is a survey of "F n.m.r. spectroscopy.) 

(b) Pople, Schneider, and Bernstein, High Resolution Nuclear Magnetic 
Resonance, McGraw-Hill, New York, 1959. 

(c) Coeio, Structure of High Resolution NMR Spectra, Academic Press, New 
York, 1966. 

(d) Roberts, Nuclear Magnetic Resonance, McGraw-Hill, New York, 1959. 

(e) Jaokman, Applications of Nuclear Magnetic Resonance Spectroscopy in 
Organic Chemistry, Pergamon Press, Ltd., Oxford, 1959. 

4. Filipovioh and Tiers, J. Phys. Ohem., 1959, 68, 761. 

5. Dyer and Lee, Trans. Faraday Soc, 1966, 62, 257. 

6. Barlow, unpublished observations. 

7. Abraham and Catalli, Mol. Phys., 1965, 9, 67. 

8. Dean and Lee, Trans. Faraday Soc, 1968, 64, 1409. 

9. Fbankiss, J. Phys. Chem., 1963, 67, 752. 

10. Williamson, Li, Hall, and Swageb, J. Amer. Chem. Soc., 1966, 88, 5678. 

11. Mobeland and Brey, J. Chem. Phys., 1966, 45, 803. 

12. Cottle, Stafford, and Stone, Spectrochim. Acta, 1961, 11, 968. 

13. Jolly, Bruce, and Stone, J. Chem. Soc, 1965, 6830. 

14. Barlow, Chem. Comm., 1966, 703. 

15. Reuben, Shvo, and Demtbl, J. Amer. Chem. Soc, 1965, 87, 3995. 

16. Bbaudbt and Baldeschwtelbb, J. Mol. Spectros., 1962, 9, 30. 

17. Bourn, Gillies, and Randall, Proc Chem. Soc, 1963, 200; Boden, Emsley, 
Fbeney, and Sutolipfe, Mol. Phys., 1964, 8, 133; Lawbenson, J. Chem. Soc, 
1965, 1117; Lawbenson and Jones, J. Chem. Soc (B), 1967, 797; Fields, Lee, 
and Mowthobpe, J. Chem. Soc. (B), 1968, 308; Bablow, Green, Haszeldine, 
and Higson, J. Chem. Soc (B), 1966, 1025; Chambers and Cunningham, J. 
Chem. Soc (C), 1967, 2185; Callander, Cob, Matough, Moonby, Uff, and 
Winson, Chem. Comm., 1966, 820; Chambers and Chtvebs, J. Chem. Soc, 
1965, 3933, 4782; Green and Keskpatbiok, J. Chem. Soc. (A), 1968, 483. 

18. Lawrenson, J. Chem. Soc, 1965, 1117. 

19. Homer and Thomas, J. Chem. Soc. (B), 1966, 141. 

20. Hopton, Rest, Roseveab, and Stone, J. Chem. Soc (A), 1966, 1326. 

21. Abraham, Macdonald, and Peppeb, J. Amer. Chem. Soc, 1968, 90, 147. 



INDEX 



Acetylenes 

perfluoroalkyl, 59 

perfluorocyclohexyne, 104 

perfluorohexa-2,4-diyne, 109 

perfluorotolan, 213 

3,3,3-trifluoropropyne, 103 

3,3,3-trifluoropropynyl derivatives of 
some elements, 105, 109, 123 
Acidity 

of bis(pentafluorophenyl)methane, 209 

of l.l-di-H-heptanuorobutanol, 74 

of fluorocarbon hydrides, 83 

of hexafluoroacetone hydrate, 91 

of pentafluorobenzene, 213 

of pentafiuorophenol, 208 

of perfluoro-t-butaaol, 167 

of perfluoropinacol, 93 

of 2,3,5,6-tetrafluoro-4-hydroxy- 
pyridine, 224 

of trifluoroacetic acid, 72 

of trifluoromethanesulphonicacid, 181 

of trifluoromethyl-substituted oxy- 
aoids of P, As, and Sb, 155, 157, 161 

of tris(pentafluorophenyl)methane, 209 
Acids 

perfiuoroalkaneoarboxylic, 70 

perfluoroalkanesulphonic, 178 

perfluoroarenecarboxylic, 211, 212, 214 

perfluoroarenesulphonic, 212 

perfluoroisonicotinic acid, 223 

perfluoronicotinio acid, 226 

trifluoromethyl-substituted oxyacids 
of P, As, and Sb, 155, 159, 161 
Aoyl hypohalites, perfluoro-, 80 
Addition reactions 

of bistrifluoromethylketene, 97 

of hexafluoroacetone, 90 

of hexafluoroazomethane, 144 

of hexafluorothioacetone, 95 

of pernuoroallenes, 57 

of perfluorobut-2-yne, 62 

of perfluoro(methylenemethylamine), 
135 

of perfluoro-olefins, 26 

of trifluoronitrosomethane, 143 



Alcohols 

perfluoro-, 165 

polyfluoro-, 73, 74, 75, 90, 93, 104, 106 
Aldehydes, perfluoro-, 73, 74, 214 
Aliphatic fluorocarbons, 7 
Alkyl halides, perfluoro-, 79 
Alkylsulphur pentafiuorides, 185 
Alkylsulphur trifluorides, 185 
Aluminium 

pentafluorophenyl derivatives of, 220 

perfluoroalkyl derivatives of, 117 
Aluminium tribromide, rearrangement of 

2,3-dibromo-l,l,3,3-tetrafluoropropene 

with, 59 
Aluminium trichloride 

cleavage and chlorination of 
perfluoro-ethers with, 162 

reaction with perfluoro-olefins, 37 

rearrangement of fluorocarbon 
epoxides with, 165 
Amides 

<x,a-difluoroacetamides, 28 

from perfluoropropene oxide, 164 

perfluoro-, 73, 75 

(Hofmann degradation), 214 
Amines 

pentaflnoroaniline, 211, 217 

perfluoroalkyl and -cycloalkyl, 124, 
145 

polyfluoroalkyl, 74 
Analysis 

elemental, 17, 131 

gas-liquid chromatographic, 16 

spectroscopic, 230 
Antimony 

pentafluorophenyl derivatives of, 220 

trifluoromethyl derivatives of, 157 
Aromatic compounds 

fluorocarbons, 203 

nitrogen heterocyoles, 221 
Arsenic 

cyclio fluorocarbon derivatives of, 64, 
159 

pentafluorophenyl derivatives of, 220 

perfluoroalkyl derivatives of, 157 



239 



Index 



Arylsulphur pentafluorides, 187 

Arylsulphur trifluorides, 185 

Azides 

2-amino-2-azidohexafluoropropane, 146 
azidobisfcrifluoromethylphosphine, 149 
pentafluorophenyl azide, 211 
perfluoroalkanoyl azides, 73, 76 
perfluoropropenyl azide, 127 

Basicity 

of 1,1-di-H-heptafluorobutylamine, 74 

of pentafluoroaniline, 217 

of pentafluoropyridine, 222 

of perfluoroethers, 161 

of perfluoro tertiary amines, 131 

relative of trifluoromethyl derivatives 
ofP, As, andSb, 160 
Benzotrifluoride, preparation and 

fluorination, 9 
Benzyne, perfluoro-, 213 
Bismuth, perfluoroalkyl derivatives of, 

157 
Bi8trifluoromethylamino-derivatives, 135 
Bistrifluoromethylnitroxide, 139 
Bistrifluoromethyl peroxide, 173 
Bistrifluoromethylthio-derivatives, 177 
Bistrifluoromethyl trioxide, 173 
Bond dissociation energies 

F— F, 7 

C— F, 13 

C— C, 17 

N— N, 129, 144 
Bonding, in fluorocarbon derivatives of 

transition metals, 190, 192 
Boron 

pentafluorophenyl derivatives of, 214, 
220 

perfluoroalkyl derivatives of, 113 

perfluorovinyl derivatives of, 115 

Cadmium, bispentafluorophenyl-, 213, 220 
Carbanions 

perfluoro-, 22, 33, 58, 60, 81, 82, 83, 88, 

107, 123, 127, 179, 184 
polyfluoro-, 26, 31, 32, 46, 98, 119 
Carbenes 

dichlorooarbene, 120 
difluorooarbene, 19, 21, 51, 63, 77, 96, 
103, 107, 114, 116, 121, 122, 145, 147, 
152, 165, 192 
other fluorinated carbenes, 119, 145 
Carbon, fluorination of, 1, 2 
Carbon monofluoride, 2 



Carbon tetrafluoride, 1, 2, 17 

Carbyne, trifluoromethyl-, 112 

Chlorodifluoromethane, preparation and 
pyrolysis of, 21 

Cobalt, fluorocarbon derivatives of, 187, 
220 

Cobalt trifluoride, as a fluorinating agent, 
10, 24, 205 

Copper, fluorocarbon derivatives of, 105, 
217 

Curtius rearrangement of perfluoro- 
alkanoyl azides, 73, 76 

Cyclo-addition reactions 

of hexafluoroacetone, 94, 161 

of hexafluoro-Dewar-benzene, 208 

of hexafluorothioacetone, 95 

of perfluoroacetylenes, 61, 63, 104 

of perfluoroallenes, 56, 104 

of perfluorobenzyne, 215 

of perfluoro-l,3-dienes, 49, 225 

of perfluoroketenes, 96, 98 

of per- and poly-fluoro-olefins, 46, 104, 

161, 166, 181 
of trifluoronitrosomethane, 51, 57, 141 

Defluorination reactions, 17, 132, 133, 

134, 147, 172, 204, 205, 221 
Dehalogenation of fluorocarbon 

1,2-dihalides, 23, 54, 59, 60 
Dehydrohalogenation of hydrofluoro- 

and halogenohydrofluorocarbons, 23, 

45, 54, 55, 204 
Dewar benzene, perfluoro-, 208 
Diazirines 

Bistrifluoromethyldiazirine, 146 

Difluorodiazirine, 147 
Diazoalkanes 

Bistrifluoromethyldiazomethane, 145 

Difluorodiazomethane, 143 

lH-Heptafluorodiazobutane, 73, 74 

2,2,2-Trifluorodiazoethane, 112, 119 
Diazotisation of pentafluoroaniline, 217 
Dicarboxylic acids 

preparation, 71 (aliphatic), 211, 212 
(aromatic) 

Kolbe electrolysis, 76 
Dicumenechromium, use for reductive 

defluorination of •AT,.Ai r -dinuoroamines, 

132, 134 
Diels-Alder reactions, 48, 50, 63, 94, 95, 

104, 208, 215, 225 
JV^ST-Difluoroalkylamines, 130 
Diketones, 88 



240 



Index 



Electrochemical fluorination 
description of the method, 11 
fluorination of: alcohols, 12, 161 ; 

alkane- and arene-sulphonyl halideg, 
178; alkanoyl halides, 70; amines, 12, 
129; carbon diselenide, 187; dimethyl 
selenide, 187; ethers, 161; hydro- 
carbons, 12; organo-sulphur com- 
pounds, 182; pyridine, 12 
mechanism, 12 
Electrolysis 

(see also electrochemical fluorination) 
Kolbe reaction, 2, 76 
of trifluoromethanesulphonic acid, 181 
Electronegativity 

of perfluoroalkyl groups, 74 
of the pentafluorophenyl group, 209 
of the pentafluoropyridyl group, 224 
Electrophilic attack 

on pentafluorobenzene, 210 
on perfluoroallenes, 58 
on perfluoro -olefins, 35 
Epoxides, perfluoro-, 162 
Ethers 

perfluoro-, 161, 172 
polyfluoro-, 26, 32, 89, 162, 166 

Ferrocene, use for reductive defluorina- 
tion of JV-fluoro-amines and fluoroxy- 

oompounds, 132, 134, 147, 172 
Fluon, 41 
Fluorel, 44 
Fluorination 

by the electrochemical method, 1 1 

with cobalt trifluoride, 10 

with fluorine, 7 
Fluorocarbons 

aliphatic, 7 

analysis of, 17 

aromatic, 203 

history, 1 

molecular geometry of, 13 

nomenclature of, 4 

physical properties of, 15 (aliphatic), 
207 (aromatic) 

polymers, 41 

purification of, 16 

thermal stability of, 17 (saturated 
aliphatic), 208 (aromatic) 

toxicity, 24 
Free radicals 

attack by, on hexafluoroacetone, 94 
on hexafluorobenzene, 215 



Free radicals— eont. 

attack by, on perfluoroallenes, 57 

on perfluorobut-2-yne, 62 
on perfluoro-olefins, 37 
bistrifluoromethylamino, 135 
bistrifluoromethylnitroxide, 139 
difluoroamino, 129 
directive effects during attack on 

perfluoropropene, 39 
pentafluorophenyl, 216 
perfluoroalkoxy, 86, 141, 173 
perfluoroalkyl, 19, 37, 72, 80, 81, 85, 89, 

105, 112, 129, 139, 144, 173, 184 
perfluorocycloalkyl, 18 

Germanium 

pentafluorophenyl derivatives of, 220 

trifluoromethyl derivatives of, 120 
Grignard reagents 

alkyl, reaction with perfluoro-olefins, 29 

pentafluorophenyl, 212, 221 

pentafluoropyridyl, 223 

perfluoroalkyl, 105 

perfluorovinyl, 107, 115 

phenylmagnesium bromide, reaction 
with perfluoroalkyl iodides, 106 

Heptafluoro-n-butyric acid 

preparation, 70 

properties, 72 
Hexafluoroacetone, 87 
Hexafluoroazomethane, 143 
Hexafluoroazoxy me thane, 142 
Hexafluorobenzene, 203 
Hexafluorobuta- 1 ,3-diene 

cyclobutane formation with hydro- 
carbon olefins, 51 

fluoride-initiated isomerization, 60 

polymerization of, 44 

preparation of, 23, 24 

thermal dimerization and isomerization, 
49 
Hexafluorobut-2-yne, 60, 61, 192 
Hexafluorothioacetone, 94 
History of fluorocarbon chemistry, 1 
Hofmann degradation of perfluoro- 

amides, 75 
Hunsdiecker reaction for the preparation 

of perfluoroalkyl halides, 79 
Hydrogenation 

of 3-chloro-2,4,5,6-tetrafluoropyridine, 
226 

of hexafluoroacetone, 93 



241 



Index 



Hydrogenation— cant. 

of hexafluoroazome thane, 145 
of perfluorobut-2-yne, 62 
of perfluoro-olefins, 45 
Hydrolysis 

of bistrifluoromethyl disulphide, 175 
of ^-bromobistrifluoromethylamine, 

135 
of oyanurio fluoride, 225 
of/?,/?-dichlorotrifluoropropionic acid, 

71 
of a,a -difluoroacetamidin.es, 28 
of ethyl heptafluoro-n-butyrate, 74 
of perfluoroalcohols, 165 
of perfluoroaldehydes, 74 
of perfluoroalkanoyl fluorides, 70 
of perfluoroalkylalumiuium 

compounds, 117 
of perfluoroalkyl-arseuic, -antimony, 

and -bismuth compounds, 158 
of perfluoroalkylgermanium 

compounds, 121 
of perfluoroalkyl -lead compounds, 124 
of perfluoroalkylmercury compounds, 

110, 112 
of perfluoroalkylphosphorus 

compounds, 151, 153, 154 
of perfluoroalkylselenium compounds, 

187 
of perfluoroalkylsilicon compounds, 118 
of perfluoroalkylsilver compounds, 105 
of perfluoroalkylsulphur compounds, 

175, 178, 181, 184 
of perfluoroalkyltin compounds, 123 
of perfluoroalkyl transition-metal 

compounds, 192 
of perfluoroalkylzino compounds, 108 
of perfluoroamines, 126 
of perfluorobutyrolactone, 165 
of perfluoroepoxides, 164, 165 
of perfluoro(fluoroxyalkanes), 171, 172 
of perfluoroimines, 126 
of perfluoroketones, 94 
of perfluoro-/? -sultones, 181 
of perfluorotoluene, 212 
of perfluorovinylaluminium comr 

pounds, 117 
of perfluorovinylboron compounds, 115 
of perfluoro-o-xylene, 139 
of polyfluoroalkylsilicon compounds, 

118 
of polyfluorocyclobutenes, 48 
of polyfluoroethers, 162, 166 



Hyperconjugation, negative, involving 

fluorine, 32, 83 
Hypofluorites, perfluoroalkyl, 169 

Indium, pentafluorophenyl derivatives, 

220 
Infrared spectroscopy, 210, 230 
Iridium, tetrafluoroethylene complex of, 

191 
Iron, fluorooarbon derivatives of, 187, 

210, 220 
Isocyanates, perfluoroalkyl, 73, 75, 127 
Isocyanides, trifluoromethyl isocyanide, 

148 
Isomerizations 
fluoride-initiated : 

perfluoroalk-1-enes to perfluoroalk- 

2-enes, 34; 
perfluorobuta-l,3-diene to perfluoro- 

but-2-yne, 60; 
perfluoro-olefin oxides to carbonyl 

compounds, 164; 
perfluoropenta-l,2-diene to per- 

fluoropent-2-yne, 60; 
perfluoropenta-l,4-diene to per- 
fluoropent-2-yne, 60 
of hexafluorobenzene to hexafluoro- 

Dewar-benzene, 208 
of perfluoro-W-fluorocycloalkylamines, 

132 
of perfluoro-olefin oxides to acyl 
fluorides or ketones, 87, 164 
Isotactic polyhexafluoropropylene, 44 

Ketones, perfluoro-, 96 
Ketones, perfluoro-, 87 

Lead, perfluoroalkyl derivatives of, 124 
Lead tetrakis(trifluoroacetate), 78 
Lithium, fluorooarbon derivatives of, 

102, 212 
Lithium aluminium hydride 

reaction -with carbonyl compounds, 73, 
93 

reaction with pentafluoropyridine, 223 

reaction with perfluoroalkyl iodides, 
117 

reaction with perfluoro-olefins, 45 

Magnesium, fluorocarbon derivatives of, 

105, 212 
Manganese, fluorooarbon derivatives of, 

187, 222 



242 



Index 



Mercury 

defluorination with, 132, 172 

fluorooarbon derivatives of, 109, 135, 
177, 216, 220 
Molecular geometry of perfluoroalkanes, 

13 
Molybdenum, polyfluoroalkyl derivatives 

of, 191 

Nickel, fluorooarbon derivatives of, 187 
Nitration of pentafluorobenzene, 212 
Nitrogen, fluorooarbon derivatives of, 124 
Nuclear magnetic resonance ( 18 F) 

spectroscopy, 231 
Nucleophilic attack 

on hexafluorobenzene, 210 

on AT-fluoroamines, 133 

on pentafluorophenyl derivatives, 217 

on perfluoroacetylenes, 61 

on perfluoroallenes, 58 

on perfluoroimines, 126, 135 

on perfluoroketenes, 97 

on perfluoroketones, 90 

on perfluoro-olefins, 26 

on perfluoro-olefin oxides, 164, 165, 168 

on tetrafluoro-pyridazine, -pyrimidine, 
and -pyrazine, 222 

on trifluoroiodomethane, 82 

on trifluoronitrosomethane, 142 

Olefin oxides, perfluoro-, 162 
Olefins, perfluoro-, 20 
Oxazetidines, perfluoro-, 74, 134, 141 
Oxetanes, perfluoro-, 161 
Oxidation 

of bis(trifluoromethylthio)mercury, 178 

of iodotrinuoromethylarsines, 160 

of perfluoroalkyl iodides, 86 

of perfluoro-olefins, 35, 162 

of polyfluoro-olefins, 71 

of tetrakistrifluoromethyloyolotetra- 
phosphine, 153 

of trifluoromethylphosphine, 151 

of trifluoromethylphosphonous acid, 
156 

of trifluoronitrosomethane, 142 

using peroxytrifluoroacetic acid, 78 

Palladium, fluorooarbon derivatives of. 

190, 213, 220 
Pentafluoroaniline, 211, 217 
Pentafluorophenol, 209, 211 



Pentafluorophenyl derivatives of the 

elements, 212, 220 
Pentafluorophenyl radicals, 216 
Pentafluoropyridine, 221 
Perfluoro-alcohols, 165 
Perfluoro-aldehydes, 73, 74, 211, 223 
Perfluoroalkaneoarboxylie acids and their 

derivatives, 70 
Perfluoro-alkanes and -cyoloalkanes, 7 
Perfluoro-alkenes, -alkadienes, -cyclo- 

alkenes, and -cycloalkadienes, 20 
Perfluoro-alkoxides, 165 
Perfluoroalkyl derivatives of the elements 

of aluminium, 117 

of antimony, 157 

of arsenic, 157 

of bismuth, 157 

of boron, 113 

of caesium, 34, 88, 102, 167, 179, 183 

of copper, 105 

of germanium, 120 

of lead, 124 

of lithium, 102 

of magnesium, 105 

of mercury, 109 

of nitrogen, 124 

of oxygen, 161 

of phosphorus, 148 

of potassium, 34, 81, 88, 102, 223 

of rubidium, 102 

of selenium, 187 

of silicon, 117 

of silver, 105 

of sodium, 34, 88, 102 

of sulphur, 174 

of tin, 122 

of transition metals, 187 

of zinc, 107 
Perfluoroalkyl hydrides, 83 
Perfluoroalkyl iodides, 79 
Perfluoroalkynes, 59 
Perfluoroallenes, 53 
Perfluoroamines, 124 
Perfluoro-aromatic compounds, 203 
Perfluorobenzyne, 213 
Perfluorocyolopentadiene, 23, 50 
Perfluoro-epoxides, 162 
Perfluoro-ethers, 161 
Perfluoroketenes, 96 
Perfluoroketones, 87 
Perfluoro(methylenemethylamine)-, 134 
Perfluoronitrosoalkanes, 74, 138 
Perfluoropinacol, 93, 167 



243 



Index 



Perfluorothioketenes, 96 
Perfluorothioketones, 94 
Perfluorovinyl derivatives 
of aluminium, 117 
of boron, 115 
of lithium, 103 
of magnesium, 107, 115 
of mercury, 111 
of oxygen, 168 
of sulphur, 181, 187 
of tin, 115 
Peroxytrifluoroaeetie aeid, 78 
Phosphorus, fluoroearbon derivatives of, 

64, 148, 214 
Photolysis 

of bisperfluoroalkylmercurials, 112 
of bistrifluoromethyldiazomethane, 145 
of difluorodiazirine, 147 
of hexafluoroacetone, 89 
of hexafluoroazome thane, 144 
of hexafluorobenzene, 208 
of perfluoroalkyl iodides, 85 
of tetrafluoroethylene, 20 
of 2,2,2-trifluorodiazoethane, 119 
Platinum, fluoroearbon derivatives of, 

191, 192 
Polymers 

perfluorinated polyethers, 169 
polyhexafluorobutadiene, 44 
polyhexafluorobut-2-yne, 61 
polyhexafluoropropylene, 43 
polytetrafluoroallene, 56 
polytetrafluoroethylene, 3, 42 
tetrafluoroethylene-hexafluoro- 

propylene copolymer, 43 
trifluoronitrosomethane-tetrafluoro- 

ethylene copolymer, 140 
vinylidene fluoride -hexafluoro- 
propylene copolymer, 44 
Pyrolysis 

of bisperfluoroalkylmercurials, 112 
of bisperfluoroisopropylsulphur 

difluoride, 185 
of .A/^-bistrifluoromethylcarbamyl 

fluoride, 134 
of bistrifluoromethyldiazirine, 146 
of bistrifluoromethyldiazomethane, 145 
of ^-bromobistrifluoromethylamine, 

135 
of carbon monofluoride, 2 
of carbon tetrafluoride, 17 
of difluorodiazirine, 147 
of difluoromaleic anhydride, 59 



Pyrolysis — coni. 

of difluorotristrifluoromethyl- 

phosphorane, 152 
of disodium hexafluoroglutarate, 59 
of disodium tetrafluorosuccinate, 59 
of fluoroalkylboron compounds, 120 
of hexafluoroacetone, 89 
of hexafluoroazomethane, 144 
of hexafluorobenzene, 208 
of hexafluorothioacetone dimer, 95 
of pentafluorophenylcopper, 217 
of pentakistrifluoromethylcyclo- 

pentaphosphine, 154 
of perfluoro-n-alkanes, 19 
of perfluoroalkanoyl nitrites, 73, 138 
of perfiuoro-alkoxides, 168 
of perfluoroalkylsulphur penta- and 

tetra-fluorides, 184 
of perfluoroallene dimer, 50 
of perfluorobicyclohexyl, 18 
of perflnorobuta-l,3-diene, 49 
of perfluorocyclopentadiene dimer, 50 
of perfluorocyclopropane, 19 
of perfluoro(fluoroxyalkanes), 172 
of perfluoroisopropylsilver, 105 
of perfluoro-(2-methyl-l,2-oxazetidine), 

134 
of perfluoromorpholine, 133 
of perfluoropiperidine, 132 
of perfluoropropene, 21 
of perfluoro tertiary amines, 132 
of perfluorovinylboron compounds, 115 
of polyfluoroalkylsilicon compounds, 

119 
of polyhexafluoropropylene, 43 
of polytetrafluoroethylene, 21 
of potassium perfluoroalkanecarboxy- 

lates, 34 
of potassium trifluoromethyltetra- 

fluoroborate, 116 
of sodium ^-bromoheptafluorobutyr- 

amide, 76 
of sodium chlorodifluoroacetate, 77 
of sodium perfluoroalkanecarboxylates, 

21 
of sodium trifluoroacetate, 77 
of tetrafluoroethylene, 21 
of tetrafluorosuccinic anhydride, 59 
of tetrakistrifluoromethyldiphosphine, 

154 
of tetrakistrifluoromethylhydrazine, 

144 
of tribromofluoromethane, 204 



244 



Index 



Pyrolysis — cont . 
of triehloromethyltrichlorosilane, 1 20 
of trifluoroacetic acid, 77 
of trifluoroacetyl nitrite, 138 
of trifluoromethylboron difluoride, 113 
of trifluoromethyliodotetraoarbonyl- 

iron, 192 
of trifluoromethyltri-iodogermane, 121 
of trifluoronitrosomethane, 139 
of trifluoronitrosomethane-tetrafluoro- 

ethylene copolymer, 141 
of trimethyltrifluoromethyltin, 122 

Rhenium, perfluoroalkyl derivatives of, 

188 
Rhodium, tetrafluoroethylene complexes 

of, 191 

Selenium, fluorocarbon derivatives of, 64, 

187 
Silastic LS-53, 118 
Silicon 

pentafluorophenyl derivatives of, 214, 
216 

perfluoroalkyl derivatives of, 103, 117 

polyfluoroalkyl derivatives of, 118 
Spectroscopy, i.r. and 19 F n.m.r., 230 
Sulphonic acids 

pentafluorobenzenesulphonic acid, 212 

perfluoroalkanesulphonic acids and 
their derivatives, 178 
Sulphur, fluorocarbon derivatives of, 64, 

174, 213, 220 
Sulphur tetrafluoride 

as a fluorinating agent, 60 

hydrolysis of, 184 

Teflon, 41 

Tetrafluoroethylene 
addition reactions of: 

electrophilic, 36; 

free-radical, 37, 118, 122; 

nucleophilic, 26, 34, 80 
care in manipulation of, 24 
copolymerisation of, 43, 74, 140 
cycloaddition reactions of, 20, 46, 74, 

140 



Tetrafluoroethylene — cont. 

hydrogenation of, 45 

physical properties of, 24 

polymerization of, 42 

preparation of, 21 

pyrolysis of, 21 

reaction with mercuric fluoride, 111 

reaction with transition-metal 
compounds, 190 

reaction with trifluoromethyl hypo- 
fluorite, 172 
Tetrafluorohydrazine, 129 
Thallium 

pentafluorophenyl derivatives, 213, 220 

perfluoroalkyl derivatives, 113 
Titanium, pentafluorophenyl derivatives 

of, 220 
Transition metals, fluorocarbon 

derivatives of, 187, 220 
Trifluoroacetic acid 

electrolysis of, 2, 173 

preparation and properties of, 70, 72, 
76 
Trifluoroacetic anhydride, 77 
Trifluoroiodomethane, 79 
Trifluoromethanesulphonic acid and its 

derivatives, 178, 181 
Trifluoromethyl hypofluorite, 169 
Trifluoronitrosomethane, 138 
Tungsten, tetrafluoroethyl derivative of, 

191 

UUmann reaction 

applied to 4-bromotetrafluoropyridine, 

223 
applied to pentafluorohalogeno- 
benzenes, 216 

Viton, 44 

Wittig reaction, applied to hexafluoro- 
acetone, 92 

Zinc, fluorocarbon derivatives of, 107, 
213, 220 



245 



University Chemistry Series 

Maedonald's University Chemistry Series is designed to meet the 
requirements of all chemists from the final-yea* undergraduate at 
university or technical college upwards. Each monograph emphasises 
modern chemical concepts and gives the student basic information 
within a text of modest length. Revised editions will be published 
at regular intervals in order to keep pace with research develop- 
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Editors: (Onjanic) "Professor M.E.GRUNDON, M.A., D.Phil. 
(Oxon.) New University of Ulster 

{Inorganic) B.C. SMITH, 31. A. (Ckwh.), Ph.D. (Xolt.) 
Bkkbeck College, London University 

(Physical) Professor M. W. ROBERTS, fi.Sc, Ph.D. (!!«/«*) 
Bradford U ni ve rsi ty 

Cnrbauitms in Synthesis 

1>. C. Ayres, BJSc, Ph.D., A.B.C.8. 
Westfieid College, London University 

The Allotropy ol the Elements 

W. E. Addison, B.Sc, Ph.D., F.R.I. (J. 

The 'University of Manchester Institute of Science and Technology 

Applications ol Spectroscopy to Organic Chemistry 

J. 0. I). Brand, D.8c,, Ph.D. 

Vanderbilt University, Nashville, Tennessee 

G. Clinton, B.Sc, Ph.D. 
University <jf Glasgow 

Oxyacids 

M. \\\ LMn; M. A., D.Phil. 

University of Toronto 

Radicals to Organic Chemistry 
C. J. M. Stirling, B,Sc, Ph.D. 

King's College, London University 

Introduction to Nucleic Acids and Related Natural Products 

T. L. V. I. lltrickt, BJSe., Ph.D. 
Twyfotd Laboratories, London 

Alicyclic Chemistry 

G. H. W'hitham, B.Sc, Ph.D. 

Dvson Pen-ins Uaborato.rv. Oxford University 



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